Semiconductor device and manufacturing method thereof

ABSTRACT

A semiconductor device with high aperture ratio is provided. The semiconductor device includes a transistor and a capacitor having a pair of electrodes. An oxide semiconductor layer formed over the same insulating surface is used for a channel formation region of the transistor and one of the electrodes of the capacitor. The other electrode of the capacitor is a transparent conductive film. One electrode of the capacitor is electrically connected to a wiring formed over the insulating surface over which a source electrode or a drain electrode of the transistor is provided, and the other electrode of the capacitor is electrically connected to one of the source electrode and the drain electrode of the transistor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/565,546, filed Sep. 10, 2019, now allowed, which is a continuation ofU.S. application Ser. No. 15/584,064, filed May 2, 2017, now U.S. Pat.No. 10,461,101, which is a continuation of U.S. application Ser. No.14/141,806, filed Dec. 27, 2013, now U.S. Pat. No. 9,647,010, whichclaims the benefit of foreign priority applications filed in Japan asSerial No. 2012-288973 on Dec. 28, 2012, and Serial No. 2013-049261 onMar. 12, 2013, all of which are incorporated by reference.

TECHNICAL FIELD

The present invention relates to an object, a method, a manufacturingmethod, a process, a machine, manufacture, or a composition of matter.In particular, the present invention relates to, for example, asemiconductor device, a display device, a light-emitting device, adriving method thereof, or a manufacturing method thereof. Inparticular, the present invention relates to, for example, asemiconductor device, a display device, or a light-emitting device eachincluding an oxide semiconductor, and a manufacturing method thereof.

BACKGROUND ART

In recent years, flat panel displays such as liquid crystal displays(LCDs) have been widespread. In a display device such as a flat paneldisplay, pixels are arranged in the row direction and the columndirection, and each pixel includes a transistor serving as a switchingelement, a liquid crystal element electrically connected to thetransistor, and a capacitor connected to the liquid crystal element inparallel.

A semiconductor film of the transistor is generally formed using asilicon semiconductor such as amorphous silicon or polysilicon(polycrystalline silicon).

Metal oxides having semiconductor characteristics (hereinafter referredto as oxide semiconductors) can also be used for semiconductor films oftransistors. For example, techniques for forming transistors includingzinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (seePatent Documents 1 and 2).

REFERENCES Patent Documents [Patent Document 1] Japanese PublishedPatent Application No. 2007-123861 [Patent Document 2] JapanesePublished Patent Application No. 2007-96055 DISCLOSURE OF INVENTION

In a capacitor, a dielectric film is provided between a pair ofelectrodes at least one of which is formed of the same material as agate electrode, a source electrode, a drain electrode, or the like of atransistor in many cases, and thus formed using a light-blocking filmsuch as a metal film.

With an increase in the capacitance value of a capacitor, the alignmentof liquid crystal molecules of a liquid crystal element can be keptconstant for a longer period in the state where an electric field isapplied. When the period can be made longer in a display device capableof displaying still images, the number of times of rewriting image datacan be reduced, leading to a reduction in power consumption.

One of the methods for increasing the charge capacity of a capacitor isto increase the area occupied by the capacitor in a pixel, specifically,to increase the area of a region where a pair of electrodes overlap eachother. However, when the area of the light-blocking conductive film isincreased in the display device in order to increase the area of theregion where the pair of electrodes overlap each other, the apertureratio of the pixel decreases, causing degradation of display quality ofan image.

Thus, in view of the above problems, an object of one embodiment of thepresent invention is to provide a semiconductor device or the like withhigh aperture ratio. Another object of one embodiment of the presentinvention is to provide a semiconductor device or the like including acapacitor whose charge capacity can be increased. Another object of oneembodiment of the present invention is to provide a semiconductor deviceor the like that can be manufactured with a smaller number of masks in aphotolithography step. Another object of one embodiment of the presentinvention is to provide a semiconductor device or the like with lowoff-state current. Another object of one embodiment of the presentinvention is to provide a semiconductor device or the like consumingless power. Another object of one embodiment of the present invention isto provide a semiconductor device or the like using a transparentsemiconductor layer. Another object of one embodiment of the presentinvention is to provide a semiconductor device or the like with highreliability. Another object of one embodiment of the present inventionis to provide an eye-friendly semiconductor device or the like. Anotherobject of one embodiment of the present invention is to provide a novelsemiconductor device or the like. Another object of one embodiment ofthe present invention is to provide a novel method for manufacturing asemiconductor device or the like.

Note that the description of these objects does not disturb theexistence of other objects. In one embodiment of the present invention,there is no need to achieve all the objects. Other objects will beapparent from and can be derived from the description of thespecification, the drawings, the claims, and the like.

One embodiment of the present invention relates to a semiconductordevice including a light-transmitting capacitor one electrode of whichis an oxide semiconductor layer and the other electrode of which is alight-transmitting conductive film.

One embodiment of the present invention is a semiconductor deviceincluding a transistor. The semiconductor device includes a first oxidesemiconductor layer and a second oxide semiconductor layer which areover a first insulating film; a source electrode layer and a drainelectrode layer which are electrically connected to the first oxidesemiconductor layer; a wiring electrically connected to the second oxidesemiconductor layer; a second insulating film over the first insulatingfilm, the first oxide semiconductor layer, the second oxidesemiconductor layer, the source electrode layer, the drain electrodelayer, and the wiring; a gate electrode layer overlapping the firstoxide semiconductor layer with the second insulating film interposedtherebetween; a third insulating film over the second insulating filmand the gate electrode layer; a fourth insulating film over the thirdinsulating film; and a transparent conductive film which is over thefourth insulating film and overlaps the second oxide semiconductorlayer. The semiconductor device also includes a capacitor including adielectric, at least part of the second oxide semiconductor layer as afirst electrode, and at least part of the transparent conductive film asa second electrode.

The first oxide semiconductor layer and the second oxide semiconductorlayer are preferably formed using the same material.

The first oxide semiconductor layer and the second oxide semiconductorlayer preferably have an energy gap of 2.0 eV or more.

One or more kinds of dopant selected from hydrogen, boron, nitrogen,fluorine, aluminum, phosphorus, arsenic, indium, tin, antimony, and arare gas element may be added to the second oxide semiconductor layer.

The dielectric may be formed using the second insulating film, the thirdinsulating film, and the fourth insulating film.

The dielectric may be formed using the third insulating film and thefourth insulating film.

The dielectric may be formed using the fourth insulating film.

The third insulating film preferably has a single-layer structure or alayered structure including an oxide insulating material selected fromsilicon oxide, silicon oxynitride, aluminum oxide, hafnium oxide,gallium oxide, and a Ga—Zn-based metal oxide.

The fourth insulating film preferably has a single-layer structure or alayered structure including a nitride insulating material selected fromsilicon nitride oxide, silicon nitride, aluminum nitride, and aluminumnitride oxide.

A nitride insulating material including hydrogen may be formed betweenthe first insulating film and the second oxide semiconductor layer.

The source electrode layer, the drain electrode layer, and the wiringmay be formed over the same insulating surface.

The source electrode layer, the drain electrode layer, and the wiringmay be formed using the same material.

The transparent conductive film may be electrically connected to one ofthe source electrode and the drain electrode.

Another embodiment of the present invention is a method formanufacturing a semiconductor device, including the steps of forming afirst oxide semiconductor layer and a second oxide semiconductor layerover a first insulating film; forming a source electrode layer and adrain electrode layer which are electrically connected to the firstoxide semiconductor layer and forming a wiring electrically connected tothe second oxide semiconductor layer; forming a second insulating filmover the first insulating film, the first oxide semiconductor layer, thesecond oxide semiconductor layer, the source electrode layer, the drainelectrode layer, and the wiring; forming a gate electrode layeroverlapping the first oxide semiconductor layer with the secondinsulating film interposed therebetween; forming a third insulating filmover the second insulating film and the gate electrode layer; forming afourth insulating film over the third insulating film; forming anopening in the second insulating film, the third insulating film, andthe fourth insulating film so as to reach the source electrode layer orthe drain electrode layer; and forming a transparent conductive filmover the fourth insulating film so as to be electrically connected tothe source electrode layer or the drain electrode layer through theopening, whereby forming a transistor, and a capacitor including atleast part of the second oxide semiconductor layer as a first electrode,at least part of the transparent conductive film as a second electrode,and a dielectric.

The first oxide semiconductor layer and the second oxide semiconductorlayer are preferably formed using the same material.

The first oxide semiconductor layer and the second oxide semiconductorlayer preferably have an energy gap of 2.0 eV or more.

One or more kinds of dopant selected from hydrogen, boron, nitrogen,fluorine, aluminum, phosphorus, arsenic, indium, tin, antimony, and arare gas element may be added to the second oxide semiconductor layer.

The dielectric may be formed using the second insulating film, the thirdinsulating film, and the fourth insulating film.

The dielectric may be formed using the third insulating film and thefourth insulating film by etching the second insulating film over thesecond oxide semiconductor layer.

The dielectric may be formed using the fourth insulating film by etchingthe second insulating film and the third insulating film over the secondoxide semiconductor layer.

The third insulating film preferably has a single-layer structure or alayered structure including an oxide insulating material selected fromsilicon oxide, silicon oxynitride, aluminum oxide, hafnium oxide,gallium oxide, and a Ga—Zn-based metal oxide.

The fourth insulating film preferably has a single-layer structure or alayered structure including a nitride insulating material selected fromsilicon nitride oxide, silicon nitride, aluminum nitride, and aluminumnitride oxide.

A nitride insulating film including hydrogen may be formed between thefirst insulating film and the second oxide semiconductor layer.

The source electrode layer, the drain electrode layer, and the wiringare preferably using the same material.

The source electrode layer, the drain electrode layer, and the wiringare preferably formed over the same insulating surface.

One embodiment of the present invention provides a semiconductor deviceor the like with high aperture ratio. One embodiment of the presentinvention provides a semiconductor device or the like including acapacitor whose charge capacity can be increased. One embodiment of thepresent invention provides a semiconductor device or the like that canbe manufactured with a smaller number of masks in a photolithographystep. One embodiment of the present invention provides a semiconductordevice or the like with low off-state current. One embodiment of thepresent invention provides a semiconductor device or the like consumingless power. One embodiment of the present invention provides asemiconductor device or the like using a transparent semiconductorlayer. One embodiment of the present invention provides a semiconductordevice or the like with high reliability. One embodiment of the presentinvention is to provide an eye-friendly semiconductor device. Oneembodiment of the present invention provides a method for manufacturinga semiconductor device.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 shows a top view illustrating a semiconductor device;

FIG. 2 shows a cross-sectional view illustrating a semiconductor device;

FIG. 3 shows a diagram illustrating a semiconductor device;

FIGS. 4A and 4B show circuit diagrams each illustrating a pixel of asemiconductor device;

FIGS. 5A and 5B show cross-sectional views illustrating a method formanufacturing a semiconductor device;

FIGS. 6A and 6B show cross-sectional views illustrating a method formanufacturing a semiconductor device;

FIGS. 7A and 7B show cross-sectional views each illustrating a capacitorof a semiconductor device;

FIG. 8 shows a cross-sectional view illustrating a semiconductor device;

FIGS. 9A and 9B show cross-sectional views each illustrating asemiconductor device;

FIGS. 10A and 10B show nanobeam electron diffraction patterns;

FIGS. 11A and 11B show the results of CPM measurement of an oxidesemiconductor film;

FIG. 12 shows the result of CPM measurement of a CAAC-OS film;

FIG. 13A shows a cross-sectional TEM image of an oxide semiconductorfilm and FIGS. 13B to 13D show nanobeam electron diffraction patterns;

FIG. 14A shows a plane TEM image of an oxide semiconductor film and FIG.14B shows a selected-area electron diffraction pattern;

FIGS. 15A to 15C show conceptual diagrams of electron diffractionintensity distribution;

FIG. 16 shows a nanobeam electron diffraction pattern of a quartz glasssubstrate;

FIG. 17 shows a nanobeam electron diffraction pattern of an oxidesemiconductor film;

FIGS. 18A and 18B show cross-sectional TEM images of an oxidesemiconductor film;

FIG. 19 shows the result of X-ray diffraction analysis of an oxidesemiconductor film;

FIG. 20 shows a cross-sectional TEM image of a CAAC-OS film;

FIGS. 21A to 21D show electron diffraction patterns of a CAAC-OS film;

FIG. 22 shows a cross-sectional TEM image of a CAAC-OS film;

FIG. 23A shows a cross-sectional TEM image of a CAAC-OS film and FIG.23B shows an X-ray diffraction spectrum thereof;

FIGS. 24A to 24D show electron diffraction patterns of a CAAC-OS film;

FIG. 25A shows a cross-sectional TEM image of a CAAC-OS film and FIG.25B shows an X-ray diffraction spectrum thereof;

FIGS. 26A to 26D show electron diffraction patterns of a CAAC-OS film;

FIG. 27A shows a cross-sectional TEM image of a CAAC-OS film and FIG.27B shows an X-ray diffraction spectrum thereof;

FIGS. 28A to 28D show electron diffraction patterns of a CAAC-OS film;

FIGS. 29A to 29C show top views each illustrating a semiconductordevice;

FIG. 30 shows a cross-sectional view illustrating a semiconductordevice;

FIGS. 31A and 31C show cross-sectional views and FIG. 31B shows a topview illustrating a semiconductor device;

FIG. 32 shows a block diagram illustrating a configuration of aninformation processor having a display function;

FIGS. 33A1 and 33A2 show block diagrams and FIGS. 33B1 and 33B2 showcircuit diagrams illustrating structures of a display portion of theinformation processor;

FIG. 34A shows a block diagram illustrating a structure of aninformation processor and FIG. 34B is a schematic view showing imagedata;

FIGS. 35A1 and 35A2 and FIGS. 35B1 and 35B2 show the effect of aninformation processor;

FIG. 36 shows a block diagram illustrating an information processor;

FIGS. 37A to 37C show electronic devices using a semiconductor device;

FIGS. 38A and 38B show an electronic device using a semiconductordevice; and

FIGS. 39A to 39C show electronic devices using a semiconductor device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that modes and details disclosedherein can be modified in various ways. Therefore, the present inventionis not construed as being limited to description of the embodimentsbelow.

In the structures of the present invention described below, the sameportions or portions having similar functions are denoted by the samereference numerals in different drawings, and description thereof is notrepeated. Further, the same hatching pattern is applied to portionshaving similar functions, and the portions are not especially denoted byreference numerals in some cases.

In each drawing described in this specification, the size, the filmthickness, or the region of each component is exaggerated for clarity insome cases. Therefore, embodiments of the present invention are notlimited to such scales.

The ordinal numbers such as “first” and “second” in this specificationand the like are used for convenience and do not indicate the order ofsteps or the stacking order of layers. In addition, the ordinal numbersin this specification do not denote particular names which specify thepresent invention.

Note that a voltage refers to a difference between potentials of twopoints, and a potential refers to electrostatic energy (electricpotential energy) of a unit charge at a given point in an electrostaticfield. Note that in general, a difference between a potential of onepoint and a reference potential (e.g., a ground potential) is merelycalled a potential or voltage, and a potential and a voltage are used assynonymous words in many cases. Thus, in this specification, a potentialmay be rephrased as a voltage and a voltage may be rephrased as apotential unless otherwise specified.

In this specification, in the case where etching treatment is performedafter a photolithography process, a mask formed by the photolithographyprocess is removed.

Embodiment 1

In this embodiment, a semiconductor device that is one embodiment of thepresent invention will be described with reference to drawings. Notethat in this embodiment, a liquid crystal display device will bedescribed as an example of the semiconductor device of one embodiment ofthe present invention. Note that the semiconductor device of oneembodiment of the present invention can be used for other displaydevices.

FIG. 3 illustrates an example of the semiconductor device of oneembodiment of the present invention. The semiconductor deviceillustrated in FIG. 3 includes a pixel portion 100; a first drivercircuit 104; a second driver circuit 106; m scan lines 107 which arearranged in parallel or almost in parallel to each other and whosepotentials are controlled by the first driver circuit 104; and n signallines 109 which are arranged in parallel or almost in parallel to eachother and whose potentials are controlled by the second driver circuit106. The pixel portion 100 includes a plurality of pixels 101 arrangedin a matrix. The semiconductor device also includes capacitor lines 115(not illustrated in FIG. 3). The capacitor lines 115 are arranged inparallel or almost in parallel to the scan lines 107 or the signal lines109.

Each scan line 107 is electrically connected to the n pixels 101 in thecorresponding row among the pixels 101 arranged in m rows and n columnsin the pixel portion 100. Each signal line 109 is electrically connectedto the m pixels 101 in the corresponding column among the pixels 101arranged in m rows and n columns Note that m and n are each an integerof 1 or more. Each capacitor line 115 is electrically connected to thenpixels 101 in the corresponding row among the pixels 101 arranged in mrows and n columns. Note that in the case where the capacitor lines 115are arranged in parallel or almost in parallel to the signal lines 109,each capacitor line 115 is electrically connected to the m pixels 101 inthe corresponding column among the pixels 101 arranged in m rows and ncolumns.

Note that the first driver circuit 104 may have a function of supplyinga signal for switching a transistor connected to the scan line 107; forexample, may serve as a scan line driver circuit. Further, the seconddriver circuit 106 may have a function of supplying an image signal to atransistor connected to the signal line 109; for example, may serve as asignal line driver circuit. Note that the first driver circuit 104 andthe second driver circuit 106 may supply another signal withoutlimitation to the above.

Since the liquid crystal display device is described as an example inthis embodiment, the wirings connected to the first driver circuit 104are referred to as the scan line 107 and the capacitor line 115 and thewiring connected to the second driver circuit 106 is referred to as thesignal line 109 for convenience; however, the functions of the wiringsare not limited by the names.

FIG. 1 shows a top view illustrating an example of a structure of thepixel 101 included in the above semiconductor device. Note that in FIG.1, a liquid crystal layer and one of a pair of electrodes included inthe liquid crystal element are omitted.

In the pixel 101 illustrated in FIG. 1, the scan line 107 extends in thedirection substantially perpendicular to the signal line 109 (in the rowdirection). The signal line 109 extends in the direction substantiallyperpendicular to the scan line 107 (in the column direction). Thecapacitor line 115 extends in the direction parallel to the signal line109. Note that the scan line 107 is electrically connected to the firstdriver circuit 104 (see FIG. 3) and the signal line 109 is electricallyconnected to the second driver circuit 106 (see FIG. 3).

The transistor 103 is provided in the vicinity of a region where thescan line 107 intersects the signal line 109. The transistor 103includes at least a semiconductor film 111 including a channel formationregion, a gate electrode, a gate insulating film (not illustrated inFIG. 1), a source electrode, and a drain electrode. A portion of thescan line 107 which overlaps the semiconductor film 111 functions as thegate electrode of the transistor 103. A portion of the signal line 109which overlaps the semiconductor film 111 functions as one of the sourceelectrode and the drain electrode of the transistor 103. A portion of aconductive film 113 which overlaps the semiconductor film 111 functionsas the other of the source electrode and the drain electrode of thetransistor 103. Thus, the gate electrode, the source electrode, and thedrain electrode are referred to as the scan line 107, the signal line109, and the conductive film 113, respectively, in some cases. Further,in FIG. 1, an edge of the scan line 107 is on the outer side of an edgeof the semiconductor film 111 when seen from above. Thus, the scan line107 functions as a light-blocking film for blocking the external light.For this reason, the semiconductor film 111 included in the transistoris not irradiated with light, so that variation in the electricalcharacteristics of the transistor can be reduced.

In one embodiment of the present invention, an oxide semiconductor ispreferably used for the semiconductor film 111. When a transistor ismanufactured using an oxide semiconductor under the optimum conditions,the off-state current of the transistor can be significantly reduced,leading to a reduction in the power consumption of the semiconductordevice.

Note that the transistor including an oxide semiconductor used in oneembodiment of the present invention is an n-channel transistor. Oxygenvacancies in an oxide semiconductor might generate carriers, which mightcause to degrade the electrical characteristics and reliability of thetransistor. For example, in some cases, the threshold voltage of thetransistor shifts in the negative direction, and drain current flowswhen gate voltage is 0 V. A transistor in which drain current flows whengate voltage is 0 V is referred to as a normally-on transistor, whereasa transistor in which substantially no drain current flows when gatevoltage is 0 V is referred to as a normally-off transistor.

Accordingly, when an oxide semiconductor is used for the semiconductorfilm 111, it is preferable that defects (typically oxygen vacancies) inthe oxide semiconductor film 111 be reduced as much as possible. Forexample, it is preferable that the spin density of the oxidesemiconductor film (the density of defects in the oxide semiconductorfilm) at a g-value of 1.93 in electron spin resonance spectroscopy inwhich a magnetic field is applied in parallel to the film surface bereduced to lower than or equal to the lower detection limit ofmeasurement equipment. When the defects in the oxide semiconductor filmare reduced as much as possible, the transistor 103 can be preventedfrom being normally on, leading to an improvement in the electricalcharacteristics and reliability of the semiconductor device.

The shift of the threshold voltage of a transistor in the negativedirection is caused in some cases by hydrogen (including a hydrogencompound such as water) contained in an oxide semiconductor as well asby oxygen vacancies. Part of hydrogen contained in the oxidesemiconductor contributes to the formation of donor levels to generateelectrons serving as carriers. Thus, a transistor including an oxidesemiconductor that contains hydrogen is likely to be normally on.

In view of the above, when an oxide semiconductor is used for thesemiconductor film 111, it is preferable that hydrogen in the oxidesemiconductor film 111 be reduced as much as possible. Specifically, thesemiconductor film 111 is formed so as to have a region where theconcentration of hydrogen measured by secondary ion mass spectrometry(SIMS) is lower than 5×10¹⁸ atoms/cm³, preferably lower than or equal to1×10¹⁸ atoms/cm³, more preferably lower than or equal to 5×10¹⁷atoms/cm³, and still more preferably lower than or equal to 1×10¹⁶atoms/cm³.

Furthermore, the semiconductor film 111 is preferably formed so as tohave a region where the concentration of alkali metals or alkaline earthmetals which is measured by secondary ion mass spectrometry (SIMS) islower than or equal to 1×10¹⁸ atoms/cm³, preferably lower than or equalto 2×10¹⁶ atoms/cm³. This is because an alkali metal and an alkalineearth metal might generate carriers when bonded to an oxidesemiconductor, in which case the off-state current of the transistor 103might be increased.

Further, when nitrogen is contained in the oxide semiconductor film 111,electrons serving as carriers are generated and the carrier densityincreases, so that the semiconductor film easily becomes n-type. Thus, atransistor including an oxide semiconductor which contains nitrogen islikely to be normally on. For this reason, nitrogen in the oxidesemiconductor film 111 is preferably reduced as much as possible; forexample, the semiconductor film 111 is preferably formed so as to have aregion where the concentration of nitrogen is lower than or equal to5×10¹⁸ atoms/cm³.

When such an oxide semiconductor film highly purified by reducingimpurities (such as hydrogen, nitrogen, an alkali metal, and an alkalineearth metal) as much as possible is used as the semiconductor film 111,the transistor 103 can be prevented from being normally on, so that theoff-state current of the transistor 103 can be significantly reduced.Accordingly, a semiconductor device having good electricalcharacteristics can be manufactured. Further, a semiconductor devicewith improved reliability can be manufactured.

Various experiments can prove the low off-state current of a transistorincluding a highly purified oxide semiconductor film. For example, evenwhen an element has a channel width of 1×10⁶ μm and a channel length (L)of 10 μm, the off-state current can be made less than or equal to themeasurement limit of a semiconductor parameter analyzer, i.e., less thanor equal to 1×10¹³ A, at a voltage (drain voltage) between the sourceelectrode and the drain electrode of 1 V to 10 V. In this case, it canbe found that the off-state current corresponding to a value obtained bydividing the off-state current by the channel width of the transistor islower than or equal to 100 zA/mm. Further, the off-state current wasmeasured with the use of a circuit in which a capacitor and a transistorare connected to each other and charge that flows in or out from thecapacitor is controlled by the transistor. In the measurement, a highlypurified oxide semiconductor film was used for a channel formationregion of the transistor, and the off-state current of the transistorwas measured from a change in the amount of charge of the capacitor perunit time. As a result, it was found that in the case where the voltagebetween the source electrode and the drain electrode of the transistorwas 3 V, an extremely low off-state current of several tens ofyoctoamperes per micrometer (yA/μm) was able to be obtained. Thus, itcan be said that the transistor including the highly purified oxidesemiconductor film has a significantly low off-state current.

In FIG. 1, the conductive film 113 is electrically connected to a pixelelectrode 121 through an opening 117. The pixel electrode 121 is one ofa pair of electrodes of the liquid crystal element which is formed usinga light-transmitting conductive film.

A capacitor 105 is formed using the following: a semiconductor film 119made of a light-transmitting oxide semiconductor which is used as oneelectrode, the light-transmitting pixel electrode 121 used as the otherelectrode, and a light-transmitting insulating film (not illustrated inFIG. 1) included in the transistor 103 which is used as a dielectricfilm That is, the capacitor 105 transmits light. Further, thesemiconductor film 119 used as one electrode of the capacitor 105 iselectrically connected to the capacitor line 115.

Since the capacitor 105 transmits light as described above, light canpass through a region where the liquid crystal element overlaps thecapacitor 105. Hence, a high aperture ratio of, for example, 55% ormore, or 60% or more is achieved even when the capacitor 105 occupies alarge area in the pixel 101. In addition, it is possible to obtain asemiconductor device with increased charge capacity of the capacitor.

For example, in a liquid crystal display device with high resolution,the area of a pixel itself is reduced, though there is a limitation on areduction in the pixel area because a capacitor needs to keep chargecapacity. This causes a reduction in aperture ratio in a liquid crystaldisplay device with high resolution. On the other hand, the capacitor105 shown in this embodiment transmits light; therefore, in each pixelincluding the capacitor 105, the aperture ratio can be increased whileenough charge capacity is maintained. It is typically preferable thatthe capacitor 105 be used for a high-resolution liquid crystal displaydevice with a pixel density of 200 ppi or more, or 300 ppi or more.Moreover, one embodiment of the present invention which enables anincrease in aperture ratio leads to an efficient use of light from alight source such as a backlight, and a reduction in the powerconsumption of the display device.

FIG. 2 illustrates a cross section along dashed-dotted lines A1-A2,B1-B2, and C1-C2 in FIG. 1 and a cross section of a transistor used forthe first driver circuit 104 in FIG. 3. Note that the top view of thefirst driver circuit 104 is omitted, and the cross section of the firstdriver circuit 104 is denoted by D1-D2 in FIG. 2. The transistor usedfor the first driver circuit 104 can also be used for the second drivercircuit 106.

First, description will be made on the cross-sectional structure of thepixel 101 along dashed-dotted lines A1-A2, B1-B2, and C1-C2.

A base insulating film 110 is provided over a substrate 102, and thesemiconductor films 111 and 119 are provided over the base insulatingfilm 110. The signal line 109 including one of the source electrode andthe drain electrode of the transistor 103, and the conductive film 113including the other of the source electrode and the drain electrode ofthe transistor 103 are provided over the semiconductor film 111. Thecapacitor line 115 is provided over the semiconductor film 119. A gateinsulating film 127 is provided over the semiconductor films 111 and119, the signal line 109, the conductive film 113, and the capacitorline 115, and the scan line 107 overlaps the semiconductor film 111 withthe gate insulating film 127 interposed therebetween. An insulating film129, an insulating film 131, and an insulating film 132 which functionas protective insulating films of the transistor 103 are provided overthe gate insulating film 127, the signal line 109, the semiconductorfilm 111, the conductive film 113, and the semiconductor film 119. Theinsulating films 129, 131, and 132 include the opening 117 (see FIG. 1)reaching the conductive film 113, and the pixel electrode 121 (seeFIG. 1) is provided to cover the opening.

In this embodiment, one of a pair of electrodes of the capacitor 105 isthe semiconductor film 119 that is formed over the base insulating film110 in the same step as the semiconductor film 111, the other of theelectrodes is the pixel electrode 121, and a dielectric film between thepair of electrodes is the insulating films 129, 131, and 132.

Note that a dopant may be added to the semiconductor film 119. In thecase where the semiconductor film 119 is an oxide semiconductor film,for example, one or more kinds of dopant selected from hydrogen, boron,nitrogen, fluorine, aluminum, phosphorus, arsenic, indium, tin,antimony, and a rare gas element may be added to the semiconductor film119, so that an n-type oxide semiconductor layer with increasedconductivity can be obtained. In other words, the semiconductor film 119is a conductive film and can be used as one of the electrodes of thecapacitor.

Note that it is preferable that the semiconductor film 119 serving as aconductive film have a hydrogen concentration higher than that in thesemiconductor film 111. In the semiconductor film 119, the hydrogenconcentration measured by secondary ion mass spectrometry (SIMS) isgreater than or equal to 8×10¹⁹ atoms/cm³, preferably greater than orequal to 1×10²⁰ atoms/cm³, and more preferably greater than or equal to5×10²⁰ atoms/cm³. In the semiconductor film 111, the hydrogenconcentration measured by SIMS is less than or equal to 5×10¹⁹atoms/cm³, preferably less than or equal to 5×10¹⁸ atoms/cm³, morepreferably less than or equal to 1×10¹⁸ atoms/cm³, still more preferablyless than or equal to 5×10¹⁷ atoms/cm³, and further preferably less thanor equal to 1×10¹⁶ atoms/cm³.

The resistivity of the semiconductor film 119 serving as a conductivefilm is lower than that of the semiconductor film 111. The resistivityof the semiconductor film 119 is preferably greater than or equal to1×10⁻⁸ times and less than or equal to 1×10⁻¹ times the resistivity ofthe semiconductor film 111. The resistivity of the semiconductor film119 is typically greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴Ωcm, preferably greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹Ωcm.

Note that the semiconductor film 119 can be formed using a materialdifferent from that of the semiconductor film 111 or the semiconductorfilm 231. In other words, the semiconductor film 119 can be formed in aprocess different from that of the semiconductor film 111 or thesemiconductor film 231.

Note that a semiconductor film that is formed in a manner similar tothat of the semiconductor film 119 can be used for forming a resistor.Further, a protective circuit can be formed using the resistor. Theprotective circuit protects against damage from static electricity orthe like.

Next, description will be made on the structure of the transistorprovided in the first driver circuit 104.

A conductive film 241 is provided over the substrate 102, and the baseinsulating film 110 is provided over the substrate 102 and theconductive film 241. The semiconductor film 231 is provided over thebase insulating film 110 and overlaps the conductive film 241. A wiring229 serving as one of a source electrode and a drain electrode of thetransistor 223 and a wiring 233 serving as the other of the sourceelectrode and the drain electrode of the transistor 223 are providedover the semiconductor film 231. The gate insulating film 127 isprovided over the semiconductor film 231 and the wirings 229 and 233,and a gate electrode 227 is provided over a region of the gateinsulating film 127 which overlaps the semiconductor film 231. Theinsulating films 129, 131, and 132 which function as protectiveinsulating films of the transistor 223 are provided over the gateinsulating film 127 and the gate electrode 227. Note that the transistorin the first driver circuit 104 does not necessarily include theconductive film 241.

The transistor 223 includes the conductive film 241 which overlaps thegate electrode 227 with the semiconductor film 231 interposedtherebetween, so that it is possible to reduce variation in gate voltageat which an on-current rises at different drain voltages. It is alsopossible to control a current flowing between the wiring 229 and thewiring 233 at a surface of the semiconductor film 231 facing theconductive film 241, whereby variation in the electrical characteristicsbetween transistors can be reduced. Further, the conductive film 241allows the semiconductor film 231 to be less influenced by a change inambient electric field, leading to an improvement in the reliability ofthe transistor. Further, when the potential of the conductive film 241is equal to or almost equal to the minimum potential (Vss; for example,the potential of the wiring 229 in the case where the potential of thewiring 229 is a reference potential), variation in the threshold voltageof the transistor can be reduced and the reliability of the transistorcan be improved.

Note that the insulating film provided over the gate insulating film127, the scan line 107, and the gate electrode 227 is not limited to theaforementioned three-layer film, and may have one-, two-, or four- ormore layer structure.

Then, the components of the above structure will be described in detail.

There is no particular limitation on the material and the like of thesubstrate 102 as long as the material has heat resistance enough towithstand at least heat treatment performed in a manufacturing processof the semiconductor device. Examples of the substrate are a glasssubstrate, a ceramic substrate, and a plastic substrate, and as theglass substrate, an alkali-free glass substrate such as a bariumborosilicate glass substrate, an aluminoborosilicate glass substrate, oran aluminosilicate glass substrate is preferably used. Alternatively, anon-light-transmitting substrate such as a stainless steel alloysubstrate may be used, in which case a surface of the substrate ispreferably provided with an insulating film. As the substrate 102, anyof the following may alternatively be used: a quartz substrate, asapphire substrate, a single crystal semiconductor substrate, apolycrystalline semiconductor substrate, a compound semiconductorsubstrate, a silicon on insulator (SOI) substrate, and the like.

The base insulating film 110 is formed to have a single-layer structureor a layered structure using, for example, any of insulating materialssuch as silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, and aGa—Zn-based metal oxide. Part of the base insulating film 110 has athickness of 30 nm to 500 nm, preferably 150 nm to 400 nm.

It is preferable that an oxide semiconductor film be used as thesemiconductor film 111, the semiconductor film 119, and thesemiconductor film 231. The oxide semiconductor film can be amorphous,single-crystalline, or polycrystalline. The thickness of part of thesemiconductor film 111 is greater than or equal to 1 nm and less than orequal to 100 nm, preferably greater than or equal to 1 nm and less thanor equal to 50 nm, more preferably greater than or equal to 1 nm andless than or equal to 30 nm, and still more preferably greater than orequal to 3 nm and less than or equal to 20 nm.

Note that a light-blocking film can be provided under the baseinsulating film 110 so as to hide a channel region of the semiconductorfilm 111. The light-blocking film may be formed at the same time as theconductive film 241, for example.

The oxide semiconductor that can be used for the semiconductor films111, 119, and 231 has an energy gap of 2 eV or more, preferably 2.5 eVor more, and more preferably 3 eV or more, and less than 3.9 eV,preferably less than 3.7 eV, and more preferably less than 3.5 eV. Theuse of such an oxide semiconductor having a wide energy gap can reducethe off-state current of the transistor 103. In addition, such an oxidesemiconductor has high visible-light transmittance and can be used forone electrode of the capacitor 105, so that the capacitor 105 transmitslight, resulting in an increase in the aperture ratio of a pixel in aliquid crystal display device or the like.

When the oxide semiconductor film has n-type conductivity, the opticalband gap of the oxide semiconductor film can be greater than or equal to2.4 eV and less than or equal to 3.1 eV, or greater than or equal to 2.6eV and less than or equal to 3.0 eV. In the case where an oxidesemiconductor film used as the semiconductor film 119 is, for example,an In—Ga—Zn-based metal oxide film with an atomic ratio ofIn:Ga:Zn=1:1:1, the oxide semiconductor film has an optical band gap of3.15 eV. Further, an indium tin oxide used for the pixel electrode 121or the like has an optical band gap of 3.7 eV to 3.9 eV. Accordingly,the highest energy visible light and ultraviolet light which passthrough the pixel electrode 121 can be absorbed in the semiconductorfilm 119. Since damage of the highest energy visible light andultraviolet light on eyes has been concerned, it can be said that thesemiconductor device including the light-transmitting capacitor 105 inthe pixel 101 is an eye-friendly device. Note that the capacitor 105does not need to overlap all the area of the pixel 101. The highestenergy visible light and ultraviolet light can be absorbed when thecapacitor 105 overlaps at least part of the pixel 101.

The oxide semiconductor used for the semiconductor films 111, 119, and231 preferably contains at least indium (In) or zinc (Zn).Alternatively, the oxide semiconductor preferably contains both In andZn. In order to reduce variation in the electrical characteristics ofthe transistors including the oxide semiconductor, the oxidesemiconductor preferably contains one or more stabilizers in addition toone of or both In and Zn.

Examples of the stabilizers are gallium (Ga), tin (Sn), hafnium (Hf),aluminum (Al), and zirconium (Zr). Other examples of the stabilizers arelanthanoids such as lanthanum (La), cerium (Ce), praseodymium (Pr),neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium(Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm),ytterbium (Yb), and lutetium (Lu).

For the oxide semiconductor that can be used for the semiconductor films111, 119, and 231, the following can be used for example: an indiumoxide; a tin oxide; a zinc oxide; an oxide containing two kinds ofmetals, such as an In—Zn-based oxide, a Sn—Zn-based oxide, anAl—Zn-based oxide, a Zn—Mg-based oxide, a Sn—Mg-based oxide, anIn—Mg-based oxide, or an In—Ga-based oxide; an oxide containing threekinds of metals, such as an In—Ga—Zn-based oxide (also referred to asIGZO), an In—Al—Zn-based oxide, an In—Sn—Zn-based oxide, aSn—Ga—Zn-based oxide, an Al-Ga—Zn-based oxide, a Sn—Al—Zn-based oxide,an In—Hf—Zn-based oxide, an In—Zr—Zn-based oxide, an In—Ti—Zn-basedoxide, an In—Sc—Zn-based oxide, an In—Y—Zn-based oxide, anIn—La—Zn-based oxide, an In—Ce—Zn-based oxide, an In—Pr—Zn-based oxide,an In—Nd—Zn-based oxide, an In—Sm—Zn-based oxide, an In—Eu—Zn-basedoxide, an In—Gd—Zn-based oxide, an In—Tb—Zn-based oxide, anIn—Dy—Zn-based oxide, an In—Ho—Zn-based oxide, an In—Er—Zn-based oxide,an In—Tm—Zn-based oxide, an In—Yb—Zn-based oxide, or an In—Lu—Zn-basedoxide; or an oxide containing four kinds of metals, such as anIn—Sn—Ga—Zn-based oxide, an In—Hf—Ga—Zn-based oxide, anIn—Al—Ga—Zn-based oxide, an In—Sn—Al—Zn-based oxide, anIn—Sn—Hf—Zn-based oxide, or an In—Hf—Al—Zn-based oxide.

Here, an “In—Ga—Zn-based oxide” means an oxide containing In, Ga, and Znas its main components and there is no particular limitation on theratio of In, Ga, and Zn. Further, the In—Ga—Zn-based oxide may contain ametal element other than In, Ga, and Zn.

Alternatively, a material represented by InMO₃(ZnO)_(m) (m>0) may beused as an oxide semiconductor. Note that M represents one or more metalelements selected from Ga, Fe, Mn, and Co, or the above element as astabilizer.

For example, an In—Ga—Zn-based metal oxide with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3), In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), orIn:Ga:Zn=3:1:2 (=1/2:1/6:1/3). Alternatively, an In—Sn—Zn-based metaloxide with an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3),In:Sn:Zn=2:1:3 (=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8) may beused. Note that the proportion of each atom in the atomic ratio of themetal oxide varies within a range of ±20% as an error.

Note that without limitation to the materials given above, a materialwith an appropriate atomic ratio depending on semiconductorcharacteristics and electrical characteristics (field-effect mobility,threshold voltage, and the like) may be used. Further, it is preferableto appropriately set the carrier density, the impurity concentration,the defect density, the atomic ratio of a metal element and oxygen, theinteratomic distance, the density, or the like in order to obtainnecessary semiconductor characteristics. For example, high field-effectmobility is achieved relatively easily in the case of using an In—Sn—Znoxide. Also in the case of using an In—Ga—Zn-based oxide, field-effectmobility can be increased by reducing the defect density in a bulk.

The signal line 109, the conductive film 113, the capacitor line 115,and the wirings 229 and 233 are preferably formed using a low-resistantmetal film in order to reduce resistance loss. The signal line 109, theconductive film 113, the capacitor line 115, and the wirings 229 and 233can be formed to have a single-layer structure or a layered structureusing any of metal materials such as molybdenum (Mo), titanium (Ti),tungsten (W), tantalum (Ta), aluminum (Al), copper (Cu), chromium (Cr),neodymium (Nd), and scandium (Sc), or an alloy material which containsany of these materials as its main component.

The signal line 109, the conductive film 113, the capacitor line 115,and the wirings 229 and 233 may have, for example, a single-layerstructure using aluminum containing silicon, a two-layer structure inwhich titanium is stacked over aluminum, a two-layer structure in whichtitanium is stacked over titanium nitride, a two-layer structure inwhich tungsten is stacked over titanium nitride, a two-layer structurein which tungsten is stacked over tantalum nitride, a two-layerstructure in which copper is stacked over a copper-magnesium-aluminumalloy, and a three-layer structure in which titanium nitride, copper,and tungsten are stacked in this order.

The signal line 109, the conductive film 113, the capacitor line 115,and the wirings 229 and 233 are preferably formed using, for example,aluminum or copper which is a low-resistant material. When aluminum orcopper is used, signal delay is reduced and the display quality can beimproved. Note that aluminum has low heat resistance, which tends tocause defects due to a hillock, a whisker, or migration. In order toprevent migration of aluminum, a metal material having a higher meltingpoint than aluminum, such as molybdenum, titanium, or tungsten, ispreferably stacked over aluminum. Also when copper is used, in order toprevent a defect due to migration and diffusion of copper elements, ametal material having a higher melting point than copper, such asmolybdenum, titanium, or tungsten, is preferably stacked over copper.

The signal line 109, the conductive film 113, the capacitor line 115,and the wirings 229 and 233 can also be formed using alight-transmitting conductive material that can be used for the pixelelectrode 121. Note that in the case where the semiconductor device ofone embodiment of the present invention is a reflective display device,a non-light-transmitting conductive material can be used for the pixelelectrode 121 or the substrate 102.

The gate insulating film 127 is formed to have a single-layer structureor a layered structure using, for example, any of insulating materialssuch as silicon oxide, silicon oxynitride, silicon nitride oxide,silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, and aGa—Zn-based metal oxide. In order to improve the characteristics of theinterface between the gate insulating film 127 and the oxidesemiconductor film 111, a region in the gate insulating film 127 whichis in contact with at least the semiconductor film 111 is preferablyformed using an insulating film containing oxygen.

Further, it is possible to prevent outward diffusion of oxygen from theoxide semiconductor film 111 and entry of hydrogen, water, or the likeinto the oxide semiconductor film from the outside by using aninsulating film having a barrier property against oxygen, hydrogen,water, and the like for the gate insulating film 127. Examples of theinsulating film having a barrier property against oxygen, hydrogen,water, and the like are an aluminum oxide film, an aluminum oxynitridefilm, a gallium oxide film, a gallium oxynitride film, an yttrium oxidefilm, an yttrium oxynitride film, a hafnium oxide film, a hafniumoxynitride film, and a silicon nitride film.

The gate insulating film 127 may be formed using a high-k material suchas hafnium silicate (HfSiO_(x)), hafnium silicate containing nitrogen(HfSi_(x)O_(y)N_(z)), hafnium aluminate containing nitrogen(HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, in which casegate leakage current of the transistor 103 can be reduced.

The gate insulating film 127 preferably has the following layeredstructure. It is preferable that a silicon nitride film having fewdefects be provided as a first silicon nitride film, a silicon nitridefilm from which little hydrogen and ammonia are released be provided asa second silicon nitride film over the first silicon nitride film, andany of the insulating films containing oxygen listed as those used forthe gate insulating film 127 be provided over the second silicon nitridefilm.

The second silicon nitride film preferably used is a silicon nitridefilm which releases hydrogen molecules less than 5×10²¹ molecules/cm³,preferably less than or equal to 3×10²¹ molecules/cm³, and morepreferably less than or equal to 1×10²¹ molecules/cm³, and releasesammonia molecules less than 1×10²² molecules/cm³, preferably less thanor equal to 5×10²¹ molecules/cm³, and more preferably less than or equalto 1×10²¹ molecules/cm³ by thermal desorption spectroscopy. When thefirst silicon nitride film and the second silicon nitride film are usedas part of the gate insulating film 127, the gate insulating film 127can be a film which has few defects and from which little hydrogen andammonia are released. Thus, it is possible to reduce the amount ofhydrogen and nitrogen contained in the gate insulating film 127 whichenter the semiconductor film 111.

In the case where the trap levels (also referred to as interface states)are present at the interface between an oxide semiconductor film and agate insulating film or in the gate insulating film in a transistorincluding an oxide semiconductor, a shift of the threshold voltage ofthe transistor, typically, a negative shift of the threshold voltage islikely to occur. The trap levels also cause an increase in thesubthreshold swing (S value) showing a gate voltage needed for changingthe drain current by an order of magnitude when the transistor is turnedon. There is also a problem in that the electrical characteristics donot vary uniformly but are different among transistors. Therefore, whena silicon nitride film having few defects is used as a gate insulatingfilm and an insulating film containing oxygen is provided in contactwith the semiconductor film 111, a negative shift of the thresholdvoltage can be reduced and an increase in S value can be minimized.

The thickness of the gate insulating film 127 is greater than or equalto 5 nm and less than or equal to 400 nm, preferably greater than orequal to 10 nm and less than or equal to 300 nm, and more preferablygreater than or equal to 50 nm and less than or equal to 250 nm.

The scan line 107, the gate electrode 227, and the conductive film 241can be formed to have a single-layer structure or a layered structureusing any of the materials that can be used for the signal line 109, theconductive film 113, the capacitor line 115, and the wirings 229 and233.

Further, the scan line 107, the gate electrode 227, and the conductivefilm 241 can be partly formed using a metal oxide containing nitrogen,specifically, an In—Ga—Zn-based oxide containing nitrogen, anIn—Sn-based oxide containing nitrogen, an In—Ga-based oxide containingnitrogen, an In—Zn-based oxide containing nitrogen, a Sn-based oxidecontaining nitrogen, an In-based oxide containing nitrogen, or a metalnitride (InN, SnN, or the like) film. These materials each have a workfunction higher than or equal to 5 eV (electron volts). In the casewhere the semiconductor film 111 of the transistor 103 is formed usingan oxide semiconductor, the use of a metal oxide containing nitrogen forthe scan line 107 (the gate electrode of the transistor 103) allows thethreshold voltage of the transistor 103 to shift in the positivedirection, i.e., the transistor can have normally-off characteristics.For example, in the case where an In—Ga—Zn-based oxide containingnitrogen is used, it is possible to use an In—Ga—Zn-based oxide having anitrogen concentration at least higher than that in the semiconductorfilm 111, specifically an In—Ga—Zn-based oxide having a nitrogenconcentration of 7 at. % or higher can be used.

The insulating films 129 and 131 can be formed to have a single-layerstructure or a layered structure using, for example, any of oxideinsulating materials such as silicon oxide, silicon oxynitride, aluminumoxide, hafnium oxide, gallium oxide, and a Ga—Zn-based metal oxide.

Part of the insulating film 129 has a thickness of 5 nm to 150 nm,preferably 5 nm to 50 nm, and more preferably 10 nm to 30 nm. Part ofthe insulating film 131 has a thickness of 30 nm to 500 nm, preferably150 nm to 400 nm.

Further, one or both of the insulating films 129 and 131 is preferablyan oxide insulating film in which the oxygen content is higher than thatin the stoichiometric composition. In that case, oxygen can be preventedfrom being released from the oxide semiconductor film, and the oxygencontained in an oxygen excess region can be diffused into the oxidesemiconductor film through the gate insulating film 127 to fill oxygenvacancies. For example, the oxygen vacancies in the oxide semiconductorfilm can be filled by using an oxide insulating film having thefollowing feature: the number of oxygen molecules released from theoxide insulating film by heat treatment at a temperature higher than orequal to 100° C. and lower or equal to 700° C., preferably higher thanor equal to 100° C. and lower than or equal to 500° C. is greater thanor equal to 1.0×10¹⁸ molecules/cm³ when measured by thermal desorptionspectroscopy (hereinafter referred to as TDS). Note that an oxideinsulating film partly including a region in which the oxygen content ishigher than that in the stoichiometric composition (oxygen excessregion) may be used as one or both of the insulating films 129 and 131.When such an oxygen excess region is present in a region overlapping atleast the semiconductor film 111, oxygen is prevented from beingreleased from the oxide semiconductor film and the oxygen contained inthe oxygen excess region can be diffused into the oxide semiconductorfilm to fill oxygen vacancies.

In the case where the insulating film 131 is an oxide insulating film inwhich the oxygen content is higher than that in the stoichiometriccomposition, the insulating film 129 is preferably an oxide insulatingfilm through which oxygen penetrates. Oxygen which enters the insulatingfilm 129 from the outside does not completely penetrate through theinsulating film 129 to be diffused and part thereof remains in theinsulating film 129. Further, there is oxygen which is contained in theinsulating film 129 from the first and is diffused from the insulatingfilm 129 to the outside. Thus, the insulating film 129 preferably has ahigh coefficient of diffusion of oxygen.

Further, one or both of the insulating films 129 and 131 is preferablyan insulating film having a barrier property against nitrogen. Forexample, a dense oxide insulating film can have a barrier propertyagainst nitrogen. Specifically, it is preferable to use an oxideinsulating film which can be etched at a rate less than or equal to 10nm per minute when the temperature is 25° C. and 0.5 wt % of fluoricacid is used.

In the case where an oxide insulating film containing nitrogen, such asa silicon oxynitride film or a silicon nitride oxide film, is used asone or both of the insulating films 129 and 131, the nitrogen-containingoxide insulating film is preferably formed so as to have a region wherethe nitrogen concentration measured by secondary ion mass spectrometry(SIMS) is higher than or equal to the lower limit of measurement by SIMSand lower than 3×10²⁰ atoms/cm³, preferably higher than or equal to1×10¹⁸ atoms/cm³ and lower than or equal to 1×10²⁰ atoms/cm³. In thatcase, the amount of nitrogen which enters the semiconductor film 111included in the transistor 103 can be reduced and the number of defectsin the nitrogen-containing oxide insulating film itself can be reduced.

The insulating film 132 can be formed using a nitride insulatingmaterial such as silicon nitride oxide, silicon nitride, aluminumnitride, or aluminum nitride oxide to have a single-layer structure or alayered structure.

As the insulating film 132, a nitride insulating film with a lowhydrogen content may be provided. The nitride insulating film is asfollows, for example: the number of hydrogen molecules released from thenitride insulating film is less than 5.0×10²¹ molecules/cm³, preferablyless than 3.0×10²¹ molecules/cm³, and more preferably less than 1.0×10²¹molecules/cm³ when measured by TDS in which heat treatment is performedat a film surface temperature of higher than or equal to 100° C. andlower or equal to 700° C., preferably higher than or equal to 100° C.and lower than or equal to 500° C.

Part of the insulating film 132 preferably has a thickness large enoughto prevent entry of impurities such as hydrogen and water from theoutside. For example, the thickness of the insulating film 132 isgreater than or equal to 50 nm and less than or equal to 200 nm,preferably greater than or equal to 50 nm and less than or equal to 150nm, and more preferably greater than or equal to 50 nm and less than orequal to 100 nm. When the insulating film 132 is provided, impuritiessuch as carbon are blocked by the insulating film 132 so as to minimizethe transfer of impurities to the semiconductor film 111 of thetransistor 103 and the semiconductor film 231 of the transistor 223;thus, variation in the electrical characteristics of the transistors canbe reduced.

In the case where a one-layer insulating film is provided over the gateinsulating film 127, the scan line 107, and the gate electrode 227, theinsulating film 131 is preferably provided. In the case where atwo-layer insulating film is provided, the insulating film 131 and theinsulating film 132 are preferably provided in this order from the sideof the semiconductor film.

An oxide insulating film formed by a chemical vapor deposition (CVD)method using an organosilane gas, typically a silicon oxide film, may beincluded in the insulating films formed between the gate insulating film127, the scan line 107, and the gate electrode 227, and the pixelelectrode 121, the conductive film 241, and the wirings and the likethat can be formed at the same time as the pixel electrode 121 and theconductive film 241.

The silicon oxide film can be formed to a thickness of 300 nm to 600 nm.As the organosilane gas, any of the following silicon-containingcompound can be used: tetraethyl orthosilicate (TEOS) (chemical formula:Si(OC₂H₅)₄); tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄);tetramethylcyclotetrasiloxane (TMCTS); octamethylcyclotetrasiloxane(OMCTS); hexamethyldisilazane (HMDS); triethoxysilane (SiH(OC₂H₅)₃);trisdimethylaminosilane (SiH(N(CH₃)₂)₃); or the like.

When the silicon oxide film is formed by a CVD method using anorganosilane gas, the planarity of a surface of an element portion overthe substrate 102 can be improved. As a result, without a planarizationfilm formed using an organic resin, alignment disorder of the liquidcrystal molecules can be reduced, light leakage can be reduced, andcontrast can be enhanced. It is needless to say that an organic resinfilm may be used instead of the silicon oxide film, or a stack of thesilicon oxide film and an organic resin film may be used.

The pixel electrode 121 can be formed using a light-transmittingconductive material such as indium tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, indium tin oxide containing titaniumoxide, indium zinc oxide, or indium tin oxide to which silicon oxide isadded.

Then, the components in the pixel 101 shown in this embodiment will bedescribed in detail.

FIG. 4A shows an example of a circuit diagram of the aforementionedpixel 101. The pixel 101 includes the transistor 103, the capacitor 105,and a liquid crystal element 108. The gate electrode of the transistor103 is electrically connected to the scan line 107, one of the sourceelectrode and the drain electrode of the transistor 103 is electricallyconnected to the signal line 109, and the other of the source electrodeand the drain electrode is electrically connected to one electrode ofthe capacitor 105 and one electrode (pixel electrode) of the liquidcrystal element 108. The other electrode of the capacitor 105 iselectrically connected to the capacitor line 115, and the otherelectrode (counter electrode) of the liquid crystal element 108 iselectrically connected to a wiring for supplying a counter potential tothe counter electrode.

The liquid crystal element 108 controls transmission or non-transmissionof light by an optical modulation action of a liquid crystal which issandwiched between a substrate provided with the transistor 103 and thepixel electrode and a counter substrate (e.g., a substrate provided withthe counter electrode). Note that the optical modulation action of aliquid crystal is controlled by an electric field applied to the liquidcrystal (including a vertical electric field, and an oblique electricfield). Note that in the case where the pixel electrode and the counterelectrode (also referred to as a common electrode) are provided over thesame substrate, a horizontal electric field is applied to a liquidcrystal.

FIG. 4B shows an example of a detailed circuit diagram of the pixel 101.As illustrated in FIG. 4B and FIG. 2, the transistor 103 includes thescan line 107 including the gate electrode, the signal line 109including one of the source electrode and the drain electrode, and theconductive film 113 including the other of the source electrode and thedrain electrode.

The semiconductor film 119 connected to the capacitor line 115 serves asone electrode of the capacitor 105. The pixel electrode 121 connected tothe conductive film 113 including the other of the source electrode andthe drain electrode serves as the other electrode of the capacitor 105.The insulating films 129, 131, and 132 provided between thesemiconductor film 119 and the pixel electrode 121 serve as a dielectricfilm.

The liquid crystal element 108 includes the pixel electrode 121, acounter electrode 154, and a liquid crystal layer provided between thepixel electrode 121 and the counter electrode 154.

The semiconductor film 119 in the capacitor 105 serves as the electrodeof the capacitor 105 even when the semiconductor film 119 has highresistance as the semiconductor film 111. This is because the pixelelectrode 121 can function as a gate electrode, the insulating films129, 131, and 132 can function as gate insulating films, and thecapacitor line 115 can function as a source electrode or a drainelectrode, so that the capacitor 105 can be operated in a manner similarto that of a transistor and the semiconductor film 119 can be made to bein a conductive state. Accordingly, the semiconductor film 119 isallowed to serve as one electrode of the capacitor 105.

Next, a method for manufacturing the semiconductor device illustrated inFIG. 1 and FIG. 2 will be described with reference to FIGS. 5A and 5Band FIGS. 6A and 6B.

First, the conductive film 241 is formed over the substrate 102, and thebase insulating film 110 is formed to cover the conductive film 241.

The conductive film 241 can be formed in such a manner that a conductivefilm is formed using any of the aforementioned materials, a mask isformed over the conductive film, and processing is performed using themask. The conductive film 241 can be formed by any of a variety ofdeposition methods such as an evaporation method, a CVD method, asputtering method, and a spin coating method. Note that there is noparticular limitation on the thickness of the conductive film, and thethickness of the conductive film can be determined in consideration oftime needed for the formation, desired resistivity, or the like. As themask, a resist mask formed through a photolithography process can beused, for example. The conductive film 241 can be processed by one orboth of dry etching and wet etching.

The base insulating film 110 can be formed using any of theaforementioned materials. The base insulating film 110 can be formed byany of a variety of deposition methods such as an evaporation method, aCVD method, a sputtering method, and a spin coating method.

Next, the semiconductor films 111, 119, and 231 are formed (see FIG. 1).The semiconductor films 111, 119, and 231 can be formed as follows: anoxide semiconductor film is formed using any of the aforementioned oxidesemiconductors, a mask is formed over the oxide semiconductor film, andprocessing is performed using the mask. The oxide semiconductor film canbe formed by a sputtering method, a coating method, a pulsed laserdeposition method, a laser ablation method, or the like. When using aprinting method, the semiconductor films 111 and 119 which are separatedfrom each other can be formed directly on the base insulating film 110.In the case where the oxide semiconductor film is formed by a sputteringmethod, plasma can be generated with use of a power supply device suchas an RF power supply device, an AC power supply device, or a DC powersupply device, as appropriate. As a sputtering gas, a rare gas(typically argon), an oxygen gas, or a mixed gas of a rare gas andoxygen is used as appropriate. In the case of using the mixed gas of arare gas and oxygen, the proportion of oxygen is preferably higher thanthat of a rare gas. Further, a target may be selected in accordance withthe composition of the oxide semiconductor film to be formed. As themask, a resist mask formed through a photolithography process can beused. The oxide semiconductor film can be processed by one or both ofdry etching and wet etching. The etching conditions (e.g., an etchinggas or an etching solution, etching time, and temperature) are set asappropriate depending on the material so that the oxide semiconductorfilm can be etched to have a desired shape.

The oxide semiconductor film can be formed by a CVD method. As the CVDmethod, a metal organic chemical vapor deposition (MOCVD) method or anatomic layer deposition (ALD) method may be employed.

The thermal CVD method is advantageous in that no defect due to plasmadamage is generated since it does not utilize plasma for forming a film.

Deposition by the thermal CVD method may be performed in such a mannerthat a source gas and an oxidizer are supplied to a chamber at a time,the pressure in the chamber is set to an atmospheric pressure or areduced pressure, and reaction is produced in the vicinity of thesubstrate or over the substrate.

Deposition by the ALD method may be performed in such a manner that thepressure in a chamber is set to an atmospheric pressure or a reducedpressure, source gases for reaction are sequentially introduced into thechamber, and then the sequence of the gas introduction is repeated. Forexample, two or more kinds of source gases are sequentially supplied tothe chamber by switching the respective switching valves (also referredto as high-speed valves). For example, a first source gas is introduced,an inert gas (e.g., argon or nitrogen) or the like is introduced at thesame time as or after the introduction of the first gas so that thesource gases are not mixed, and then a second source gas is introduced.Note that in the case where the first source gas and the inert gas areintroduced at a time, the inert gas serves as a carrier gas, and theinert gas may also be introduced at the same time as the introduction ofthe second source gas. Alternatively, the first source gas may beexhausted by vacuum evacuation instead of the introduction of the inertgas, and then the second source gas may be introduced. The first sourcegas is adsorbed on the surface of the substrate to form a first layer;then, the second source gas is introduced to react with the first layer;as a result, a second layer is stacked over the first layer, so that athin film is formed. The sequence of the gas introduction is repeatedplural times until a desired thickness is obtained, whereby a thin filmwith excellent step coverage can be formed. The thickness of the thinfilm can be adjusted by the number of repetitions times of the sequenceof the gas introduction; therefore, the ALD method makes it possible toaccurately adjust the thickness and thus is suitable for manufacturing aminute transistor.

For example, in the case where an InGaZnO_(X) (X>0) film is formed,trimethylindium, trimethylgallium, and dimethylzinc are used. Note thatthe chemical formula of trimethylindium is In(CH₃)₃. The chemicalformula of trimethylgallium is Ga(CH₃)₃. The chemical formula ofdimethylzinc is Zn(CH₃)₂. Without limitation to the above combination,triethylgallium (chemical formula: Ga(C₂H₅)₃) can be used instead oftrimethylgallium and diethylzinc (chemical formula: Zn(C₂H₅)₂) can beused instead of dimethylzinc.

In the case where an In—Ga—Zn—O film is formed by the ALD method, anIn(CH₃)₃ gas and an O₃ gas) are sequentially introduced plural times toform an InO₂ layer, a Ga(CH₃)₃ gas and an O₃ gas) are introduced at atime to form a GaO layer, and then a Zn(CH₃)₂ gas and an O₃ gas) areintroduced at a time to form a ZnO layer. Note that the order of theselayers is not limited to this example. A mixed compound layer such as anIn—Ga—O layer, an In—Zn—O layer, a Ga—In—O layer, a Zn—In—O layer or aGa—Zn—O layer may be formed by mixing of these gases. Note that althoughan H₂O gas which is obtained by bubbling with an inert gas such as Armay be used instead of an O₃ gas), it is preferable to use an O₃ gas),which does not contain H. Further, instead of an In(CH₃)₃ gas, anIn(C₂H₅)₃ gas may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gasmay be used. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ may be used.Furthermore, a Zn(CH₃)₂ gas may be used.

After the formation of the semiconductor films 111, 119, and 231, heattreatment is preferably performed to dehydrate or dehydrogenate theoxide semiconductor films 111, 119, and 231. The temperature of the heattreatment is typically higher than or equal to 150° C. and lower thanthe strain point of the substrate, preferably higher than or equal to200° C. and lower than or equal to 450° C., and more preferably higherthan or equal to 300° C. and lower than or equal to 450° C. Note thatthe heat treatment may be performed on the oxide semiconductor filmwhich has not been processed into the semiconductor films 111, 119, and231.

A heat treatment apparatus used in the heat treatment is not limited toan electric furnace, and may be an apparatus which heats an object usingthermal conduction or thermal radiation given by a medium such as aheated gas. For example, a rapid thermal annealing (RTA) apparatus suchas a gas rapid thermal annealing (GRTA) apparatus or a lamp rapidthermal annealing (LRTA) apparatus can be used. An LRTA apparatus is anapparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas.

The heat treatment may be performed in an atmosphere of nitrogen,oxygen, ultra-dry air (air in which the water content is less than orequal to 20 ppm, preferably less than or equal to 1 ppm, and morepreferably less than or equal to 10 ppb), or a rare gas (e.g., argon orhelium). The atmosphere of nitrogen, oxygen, ultra-dry air, or a raregas preferably does not contain hydrogen, water, and the like.Alternatively, heating may be performed in an inert gas atmospherefirst, and then in an oxygen atmosphere. The treatment time is 3 minutesto 24 hours.

Here, a dopant may be added to the semiconductor film 119 in thefollowing manner: a mask is provided in a region except thesemiconductor film 119 and one or more kinds of dopant selected fromhydrogen, boron, nitrogen, fluorine, aluminum, phosphorus, arsenic,indium, tin, antimony, and a rare gas element is added to thesemiconductor film 119 by an ion implantation method, an ion dopingmethod, or the like. Alternatively, the dopant may be added to thesemiconductor film 119 by exposing the semiconductor film to plasmacontaining the dopant instead of by using the ion implantation method orthe ion doping method. Heat treatment may be performed after the dopantis added to the semiconductor film 119. The heat treatment can beperformed as appropriate with reference to the details of heat treatmentfor dehydration or dehydrogenation of the semiconductor film 111 and thesemiconductor film 119.

Next, the signal line 109, the conductive film 113, the capacitor line115, and the wirings 229 and 233 are formed. The signal line 109, theconductive film 113, the capacitor line 115, and the wirings 229 and 233can be formed as follows: a conductive film is formed using any of theaforementioned materials, a mask is formed over the conductive film, andprocessing is performed using the mask. The formation of the mask andthe processing can be performed in a manner similar to that of theconductive film 241.

Then, the gate insulating film 127 is formed to cover the baseinsulating film 110, the semiconductor films 111, 119, and 231, thesignal line 109, the conductive film 113, the capacitor line 115, andthe wirings 229 and 233.

The gate insulating film 127 can be formed using any of theaforementioned materials by any of a variety of deposition methods suchas a CVD method and a sputtering method. In the case where gallium oxideis used for the gate insulating film 127, the gate insulating film 127can be formed by a metal organic chemical vapor deposition (MOCVD)method.

Next, the scan line 107 is formed over the semiconductor film 111 withthe gate insulating film 127 interposed therebetween, and the gateelectrode 227 is formed over the semiconductor film 231 with the gateinsulating film 127 interposed therebetween (see FIG. 5B).

The scan line 107 and the gate electrode 227 can be formed in such amanner that a conductive film is formed using any of the aforementionedmaterials, a mask is formed over the conductive film, and processing isperformed using the mask. The formation of the mask and the processingcan be performed in a manner similar to that of the conductive film 241.

Next, the insulating films 129, 131, and 132 are formed over the gateinsulating film 127, the scan line 107, and the gate electrode 227 (seeFIG. 6A). Note that the insulating films 129, 131, and 132 arepreferably formed in succession. The successive formation prevents theentry of impurities into each interface of the insulating films 129,131, and 132.

The insulating films 129, 131, and 132 can be formed using any of theaforementioned materials by any of a variety of deposition methods suchas a CVD method and a sputtering method.

The insulating film 129 can be formed using, for example, theaforementioned oxide insulating film Here, as the oxide insulating film,a silicon oxide film or a silicon oxynitride film is formed. As for theformation conditions, the substrate placed in a treatment chamber of aplasma CVD apparatus, which is vacuum-evacuated, is held at atemperature higher than or equal to 180° C. and lower than or equal to400° C., preferably higher than or equal to 200° C. and lower than orequal to 370° C., a deposition gas containing silicon and an oxidizinggas are introduced as a source gas into the treatment chamber, thepressure in the treatment chamber is set to greater than or equal to 20Pa and less than or equal to 250 Pa, preferably greater than or equal to40 Pa and less than or equal to 200 Pa, and high-frequency power issupplied to an electrode provided in the treatment chamber.

Typical examples of the deposition gas containing silicon are silane,disilane, trisilane, and silane fluoride. Examples of the oxidizing gasare oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide.

By setting the ratio of the amount of the oxidizing gas to the amount ofthe deposition gas containing silicon to 100 or higher, the hydrogencontent in the insulating film 129 can be reduced and dangling bonds inthe insulating film 129 can be reduced. Oxygen released from theinsulating film 131 is captured by the dangling bonds in the insulatingfilm 129 in some cases; thus, in the case where the dangling bonds inthe insulating film 129 are reduced, oxygen in the insulating film 131can be efficiently diffused into the semiconductor films 111 and 231through the gate insulating film 127 to fill the oxygen vacancies in theoxide semiconductor films 111 and 231. As a result, the amount ofhydrogen which enters the oxide semiconductor film can be reduced andoxygen vacancies in the oxide semiconductor film can be reduced.

In the case where the insulating film 131 is the above oxide insulatingfilm which includes an oxygen excess region or the above oxideinsulating film in which the oxygen content is higher than that in thestoichiometric composition, the insulating film 131 can be formed underthe following formation conditions. Here, as the oxide insulating film,a silicon oxide film or a silicon oxynitride film is formed. As for theformation conditions, the substrate placed in a treatment chamber of aplasma CVD apparatus, which is vacuum-evacuated, is held at atemperature higher than or equal to 180° C. and lower than or equal to260° C., preferably higher than or equal to 180° C. and lower than orequal to 230° C., a source gas is introduced into the treatment chamber,the pressure in the treatment chamber is greater than or equal to 100 Paand less than or equal to 250 Pa, preferably greater than or equal to100 Pa and less than or equal to 200 Pa, and high-frequency power thatis higher than or equal to 0.17 W/cm² and lower than or equal to 0.5W/cm², preferably, higher than or equal to 0.25 W/cm² and lower than orequal to 0.35 W/cm² is supplied to an electrode provided in thetreatment chamber.

As the source gas of the insulating film 131, a source gas which can beused for the insulating film 129 can be used.

Under the formation conditions of the insulating film 131, thehigh-frequency power having the above power density is supplied to thetreatment chamber having the above pressure; accordingly, thedecomposition efficiency of the source gas in plasma is increased,oxygen radicals are increased, and oxidation of the source gas proceeds,whereby the oxygen content in the insulating film 131 becomes higherthan that in the stoichiometric composition. However, the bondingstrength of silicon and oxygen is weak in the above substratetemperature range; therefore, part of oxygen is released by heating.Hence, it is possible to form an oxide insulating film in which theoxygen content is higher than that in the stoichiometric composition andfrom which part of oxygen is released by heating.

By increasing the thickness of the insulating film 131, a larger amountof oxygen is released by heating; thus, the insulating film 131 ispreferably formed thicker than the insulating film 129. The insulatingfilm 129 enables favorable coverage to be achieved even when theinsulating film 131 is formed thick.

In the case where a nitride insulating film with a low hydrogen contentis used as the insulating film 132, the insulating film 132 can beformed under the following formation conditions. Here, as the nitrideinsulating film, a silicon nitride film is formed. As for the formationconditions, the substrate placed in a treatment chamber of a plasma CVDapparatus, which is vacuum-evacuated, is held at a temperature higherthan or equal to 80° C. and lower than or equal to 400° C., preferablyhigher than or equal to 200° C. and lower than or equal to 370° C., asource gas is introduced into the treatment chamber, the pressure in thetreatment chamber is set to greater than or equal to 100 Pa and lessthan or equal to 250 Pa, preferably greater than or equal to 100 Pa andless than or equal to 200 Pa, and high-frequency power is supplied to anelectrode provided in the treatment chamber.

As the source gas of the insulating film 132, a deposition gascontaining silicon, a nitrogen gas, and an ammonia gas are preferablyused. Typical examples of the deposition gas containing silicon aresilane, disilane, trisilane, and silane fluoride. Further, the flowratio of nitrogen to ammonia is preferably higher than or equal to 5 andlower than or equal to 50, more preferably higher than or equal to 10and lower than or equal to 50. The use of ammonia as the source gasfacilitates decomposition of nitrogen and the deposition gas containingsilicon. This is because ammonia is dissociated by plasma energy or heatenergy, and energy generated by the dissociation contributes to thedecomposition of a bond of the deposition gas molecules containingsilicon and a bond of nitrogen molecules. Under the above conditions, itis possible to form a silicon nitride film which has a low hydrogencontent and can suppress the entry of impurities such as hydrogen andwater from the outside.

It is preferable that heat treatment be performed at least after theformation of the insulating film 131 so that excess oxygen contained inthe insulating film 129 or the insulating film 131 is diffused into theoxide semiconductor films 111 and 231 through the gate insulating film127 to reduce oxygen vacancies in the oxide semiconductor films 111 and231. Note that the heat treatment can be performed according to thedetails of heat treatment for dehydration or dehydrogenation of thesemiconductor films 111 and 231.

Next, the opening 117 (see FIG. 1) reaching the conductive film 113 isformed in part of the insulating films 129, 131, and 132 which overlapthe conductive film 113.

Then, the pixel electrode 121 is formed, whereby the semiconductordevice illustrated in FIG. 1 and FIG. 2 can be formed (see FIG. 6B). Thepixel electrode 121 is formed in such a manner that a conductive film isformed using any of the aforementioned materials in contact with theconductive film 113 through the opening 117, a mask is formed over theconductive film, and processing is performed using the mask. Theformation of the mask and the processing can be performed in a mannersimilar to that of the conductive film 241.

In the semiconductor device of one embodiment of the present invention,the structure of the capacitor can be changed as appropriate. Forexample, as in the cross-sectional view of the capacitor 105 in FIG. 7A,the gate insulating film 127 can be removed from the dielectric of thecapacitor 105. As a result, the thickness of the dielectric can bereduced and the charge capacity of the capacitor 105 can be improved.Note that the gate insulating film 127 can be partly removed byprocessing using a mask formed over the gate insulating film 127. Theformation of the mask and the processing can be performed in a mannersimilar to that of the conductive film 241. A part of the gateinsulating film 127 may be removed when the gate electrode 227 is formedusing a half-tone mask. In that case, the number of photolithographysteps can be reduced. In addition, either the insulating film 129 or theinsulating film 131 can be removed.

Further, as in the cross-sectional view of the capacitor 105 in FIG. 7B,the gate insulating films 127, 129, and 131 can be removed from thedielectric of the capacitor 105. As a result, the thickness of thedielectric can be further reduced and the charge capacity of thecapacitor 105 can be further improved.

In the capacitor 105 illustrated in FIG. 7B, the semiconductor film 119is in contact with the insulating film 132. The insulating film 132 ispreferably a nitride insulating film as described above. A large amountof nitrogen and hydrogen contained in the nitride insulating film can bediffused into the semiconductor film 119. In the case where an oxidesemiconductor is used for the semiconductor film 119, part of nitrogenand hydrogen entering the oxide semiconductor contribute to theformation of donor levels to generate carriers; accordingly, an n-typeoxide semiconductor layer is obtained. Hence, the conductivity of thesemiconductor film 119 can be increased, which makes it possible to omitthe step of doping the semiconductor film 119 with impurities, or thelike.

Further, as illustrated in FIG. 8, a nitride insulating film 118 may beprovided between the base insulating film 110 and the semiconductor film119 serving as one electrode of the capacitor 105. With such astructure, nitrogen and hydrogen can be diffused from the nitrideinsulating film 118 into the semiconductor film 119 as in FIG. 7B,resulting in an increase in the conductivity of the semiconductor film119. Note that the nitride insulating film 118 can be formed in thefollowing manner a film that can be used as the insulating film 132 isformed, a mask is formed over the film, and processing is performedusing the mask. The formation of the mask and the processing can beperformed in a manner similar to that of the conductive film 241. Such astructure of the capacitor can be combined with the structure of thecapacitor illustrated in FIG. 7A or 7B.

In the semiconductor device of one embodiment of the present invention,the shape of a transistor provided in a pixel is not limited to theshape of the transistor illustrated in FIG. 1 and FIG. 2 and can bechanged as appropriate. For example, in the transistor, one of thesource electrode and the drain electrode included in the signal line 109may have a U shape (or a C shape, a square-bracket-like shape, or ahorseshoe shape) which surrounds the conductive film including the otherof the source electrode and the drain electrode. With such a shape, anenough channel width can be ensured even when the area of the transistoris small, and accordingly, the amount of drain current flowing at thetime of conduction of the transistor (also referred to as an on-statecurrent) can be increased.

Although the transistor in the aforementioned pixel 101 is a transistorhaving one gate electrode, a transistor having two gate electrodesfacing each other with the semiconductor film 111 interposedtherebetween can be used. Note that an example of the transistorincluding two gate electrodes is the transistor illustrated in FIG. 2,which includes the gate electrode 227 and the conductive film 241 and isused for the first driver circuit 104.

In the aforementioned transistor including two gate electrodes, aconductive film is provided under the base insulating film 110 of thetransistor 103 described in this embodiment. The conductive filmoverlaps at least a channel formation region of the semiconductor film111. It is preferable that the conductive film be provided in a positionoverlapping the channel formation region of the semiconductor film 111so that the potential of the conductive film is equal to the minimumpotential of a video signal input to the signal line 109. In that case,it is possible to control a current flowing between the source electrodeand the drain electrode at a surface of the semiconductor film 111facing the conductive film, whereby variation in the electricalcharacteristics between transistors can be reduced. Further, theconductive film allows the semiconductor film 111 to be less influencedby a change in ambient electric field, leading to an improvement in thereliability of the transistor.

The above conductive film can be formed using a material and a methodsimilar to those of the conductive film 241, the scan line 107, thesignal line 109, the pixel electrode 121, or the like.

As described above, when the semiconductor film formed in the same stepas the semiconductor film included in the transistor is used for oneelectrode of the capacitor, a semiconductor device including thecapacitor with increased charge capacity can be manufactured while theaperture ratio is improved. As a result, the semiconductor device withexcellent display quality can be obtained.

Moreover, when the semiconductor film formed in the same step as thesemiconductor film included in the transistor is used for one electrodeof the capacitor, a semiconductor device which has high aperture ratioand includes the capacitor with increased charge capacity can bemanufactured without increasing the number of masks in aphotolithography step.

Furthermore, the oxide semiconductor film that is the semiconductor filmin the transistor includes reduced oxygen vacancies and impurities suchas hydrogen; accordingly, the semiconductor device of one embodiment ofthe present invention has good electrical characteristics.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

Embodiment 2

Described in this embodiment is one mode of an oxide semiconductor filmwhich can be used as a semiconductor film in the transistor and thecapacitor included in the semiconductor device described in the aboveembodiment.

An oxide semiconductor may include a non-single-crystal. Thenon-single-crystal is, for example, structured by at least one of c-axisaligned crystal (CAAC), polycrystal, microcrystal, and an amorphouspart.

An oxide semiconductor may include CAAC. Note that an oxidesemiconductor including CAAC is referred to as a CAAC-OS (c-axis alignedcrystalline oxide semiconductor).

In an image obtained with a transmission electron microscope (TEM),crystal parts can be found in the CAAC-OS in some cases. In most cases,in an image obtained with a TEM, crystal parts in the CAAC-OS each fitinside a cube whose one side is less than 100 nm. In an image obtainedwith a TEM, a boundary between the crystal parts in the CAAC-OS is notclearly observed in some cases. Further, in an image obtained with aTEM, a grain boundary in the CAAC-OS is not clearly observed in somecases. In the CAAC-OS, since a clear grain boundary does not exist,segregation of an impurity is unlikely to occur. In the CAAC-OS, since aclear boundary does not exist, high density of defect states is unlikelyto occur. In the CAAC-OS, since a clear grain boundary does not exist, areduction in electron mobility is unlikely to occur.

The CAAC-OS includes a plurality of crystal parts. In the plurality ofcrystal parts, c-axes are aligned in a direction parallel to a normalvector of a surface where the CAAC-OS is formed or a normal vector of asurface of the CAAC-OS in some cases. When the CAAC-OS is analyzed by anout-of-plane method with an X-ray diffraction (XRD) apparatus, a peakappears at 2θ of around 31° in some cases. In the case of a crystal ofInGaZnO₄, the peak at 2θ of around 31° indicates that the crystal ofInGaZnO₄ has (009) plane alignment. Further, in the CAAC-OS, a peakappears at 2θ of around 31° in some cases. In the case of a crystal ofZnGa₂O₄, the peak at 2θ of around 31° indicates that the crystal ofZnGa₂O₄ has (222) plane alignment. In the CAAC-OS, it is preferable thatthe peak appear at 2θ of around 31° and do not appear at 2θ=around 36°.

In the CAAC-OS, among crystal parts, the directions of the a-axis andthe b-axis of one crystal part are different from those of anothercrystal part, in some cases. When the CAAC-OS including a crystal ofInGaZnO₄ is analyzed with an X-ray diffractometer by an in-plane methodin which an X-ray enters a sample in the direction perpendicular to thec-axis, a peak appears at 2θ of around 56° in some cases. The peak at 2θof around 56° is derived from the (110) plane of the InGaZnO₄ crystal.Here, when 2θ is fixed at around 56°, a sample is rotated using a normalvector of a surface of the sample as an axis (ϕ axis), and analysis (ϕscan) is performed; although six peaks having symmetry appear in thecase of a single crystal oxide semiconductor in which the directions ofan a-axis and a b-axis of one crystal part are the same as those ofanother crystal part, a peak is not clearly observed in the case of theCAAC-OS.

As described above, in the CAAC-OS, c-axes are aligned, and a-axesand/or b-axes are not macroscopically aligned, in some cases.

Further, spots (luminescent spots) are shown in an electron diffractionpattern of the CAAC-OS in some cases. An electron diffraction patternobtained with an electron beam having a diameter of 10 nmϕ or smaller,or 5 nmϕ or smaller, is called a nanobeam electron diffraction pattern.

FIG. 10A shows an example of a nanobeam electron diffraction pattern ofa sample including CAAC-OS. Here, the sample is cut in the directionperpendicular to a surface where the CAAC-OS is formed and the thicknessthereof is reduced to about 40 nm. Further, an electron beam with adiameter of 1 nmϕ enters from the direction perpendicular to the cutsurface of the sample. FIG. 10A shows that spots are observed in thenanobeam electron diffraction pattern of the CAAC-OS.

In each of the crystal parts included in the CAAC-OS, a c-axis isaligned in the direction parallel to a normal vector of a surface wherethe CAAC-OS is formed or a normal vector of a surface of the CAAC-OS.Further, in each of the crystal parts, metal atoms are arranged in atriangular or hexagonal configuration when seen from the directionperpendicular to the a-b plane, and metal atoms are arranged in alayered manner or metal atoms and oxygen atoms are arranged in a layeredmanner when seen from the direction perpendicular to the c-axis. Notethat, among crystal parts, the directions of the a-axis and the b-axisof one crystal part may be different from those of another crystal part.In this specification, a term “perpendicular” includes a range from 80°to 100°, preferably from 85° to 95°. In addition, a term “parallel”includes a range from −10° to 10°, preferably from −5° to 5°.

Since the c-axes of the crystal parts included in the CAAC-OS film arealigned in the direction parallel to a normal vector of a surface wherethe CAAC-OS film is formed or a normal vector of a surface of theCAAC-OS film, the directions of the c-axes may be different from eachother depending on the shape of the CAAC-OS film (the cross-sectionalshape of the surface where the CAAC-OS film is formed or thecross-sectional shape of the surface of the CAAC-OS film). Note that thefilm deposition is accompanied with the formation of the crystal partsor followed by the formation of the crystal parts throughcrystallization treatment such as heat treatment. Hence, the c-axes ofthe crystal parts are aligned in the direction parallel to a normalvector of the surface where the CAAC-OS film is formed or a normalvector of the surface of the CAAC-OS film.

The CAAC-OS could be obtained by reducing the impurity concentration.The impurity means here an element other than the main components of theoxide semiconductor, such as hydrogen, carbon, silicon, or a transitionmetal element. In particular, an element such as silicon has a higherstrength to bond with oxygen than that of a metal element included inthe oxide semiconductor. Therefore, when the element takes oxygen awayin the oxide semiconductor, the atomic arrangement in the oxidesemiconductor is disrupted, whereby the crystallinity of the oxidesemiconductor is lowered in some cases. In addition, a heavy metal suchas iron or nickel, argon, carbon dioxide, or the like has a large atomicradius (or molecular radius), and thus disrupts the atomic arrangementin the oxide semiconductor, whereby the crystallinity of the oxidesemiconductor is lowered in some cases. Hence, the CAAC-OS is an oxidesemiconductor with a low impurity concentration. Note that the impurityincluded in the oxide semiconductor might serve as a carrier generationsource.

In the CAAC-OS, distribution of crystal parts is not necessarilyuniform. For example, in the formation process of the CAAC-OS, in thecase where crystal growth occurs from a surface side of the oxidesemiconductor, the proportion of crystal parts in the vicinity of thesurface of the oxide semiconductor is higher than that in the vicinityof the surface where the oxide semiconductor is formed in some cases.Further, when an impurity is mixed into the CAAC-OS, the crystallinityof the crystal part in a region into which the impurity is mixed islowered in some cases.

Further, the CAAC-OS can be formed by reducing the density of defectstates. In an oxide semiconductor, oxygen vacancies form defect states.The oxygen vacancies form trap states or serve as carrier generationsources when hydrogen is captured therein. In order to form the CAAC-OS,it is important to prevent oxygen vacancies from being generated in theoxide semiconductor. Thus, the CAAC-OS is an oxide semiconductor havinga low density of defect states. In other words, the CAAC-OS is an oxidesemiconductor having few oxygen vacancies.

The state in which impurity concentration is low and density of defectstates is low (the number of oxygen vacancies is small) is referred toas “highly purified intrinsic” or “substantially highly purifiedintrinsic”. A highly purified intrinsic or substantially highly purifiedintrinsic oxide semiconductor has few carrier generation sources, andthus has a low carrier density in some cases. Thus, in some cases, atransistor including the oxide semiconductor in a channel formationregion rarely has a negative threshold voltage (is rarely normally-on).A highly purified intrinsic or substantially highly purified intrinsicoxide semiconductor has a low density of defect states and accordinglyhas a low density of trap states in some cases. Thus, the transistorincluding the oxide semiconductor in the channel formation region has asmall variation in electrical characteristics and high reliability insome cases. Charges trapped by the trap states in the oxidesemiconductor take a long time to be released and may behave like fixedcharges. Thus, the transistor which includes the oxide semiconductorhaving a high density of trap states in the channel formation region hasunstable electrical characteristics in some cases.

A transistor including the highly purified intrinsic or substantiallyhighly purified intrinsic CAAC-OS has a small variation in theelectrical characteristics due to irradiation with visible light orultraviolet light.

The CAAC-OS can be formed by a sputtering method using a DC powersource, for example.

The oxide semiconductor may include polycrystal. Note that an oxidesemiconductor including polycrystal is referred to as a polycrystallineoxide semiconductor. A polycrystalline oxide semiconductor includes aplurality of crystal grains.

In a TEM image of the polycrystalline oxide semiconductor, crystalgrains can be found. In most cases, the size of a crystal grain in thepolycrystalline oxide semiconductor is greater than or equal to 2 nm andless than or equal to 300 nm, greater than or equal to 3 nm and lessthan or equal to 100 nm, or greater than or equal to 5 nm and less thanor equal to 50 nm in an image obtained with the TEM. Moreover, in theTEM image, a boundary between crystal grains can be found in thepolycrystalline oxide semiconductor in some cases. Also in the TEMimage, a grain boundary can be found in the polycrystalline oxidesemiconductor in some cases.

The polycrystalline oxide semiconductor may include a plurality ofcrystal grains, and the alignment of crystals may be different in theplurality of crystal grains. When a polycrystalline oxide semiconductoris analyzed by an out-of-plane method with use of an XRD apparatus, apeak at 2θ of around 31 degrees which shows alignment or a peak showingplural kinds of alignment appears in some cases. Further, spots areobserved in a nanobeam electron diffraction pattern of thepolycrystalline oxide semiconductor in some cases.

The polycrystalline oxide semiconductor has high crystallinity and thushas high electron mobility in some cases. Accordingly, a transistorusing the polycrystalline oxide semiconductor for a channel formationregion has high field-effect mobility. Note that there are cases inwhich an impurity is segregated at the grain boundary between thecrystals in the polycrystalline oxide semiconductor. Moreover, the grainboundary of the polycrystalline oxide semiconductor becomes a defectstate. Since the grain boundary of the polycrystalline oxidesemiconductor may serve as a carrier trap or a carrier generationsource, a transistor using the polycrystalline oxide semiconductor for achannel formation region has larger variation in electricalcharacteristics and lower reliability than a transistor using a CAAC-OSfor a channel formation region in some cases.

The polycrystalline oxide semiconductor can be formed byhigh-temperature heat treatment or laser light treatment.

The oxide semiconductor may include microcrystal. Note that an oxidesemiconductor including microcrystal is referred to as amicrocrystalline oxide semiconductor.

In an image obtained with a TEM, crystal parts cannot be found clearlyin the microcrystalline oxide semiconductor in some cases. In mostcases, the size of a crystal part included in the microcrystalline oxidesemiconductor is greater than or equal to 1 nm and less than or equal to100 nm, or greater than or equal to 1 nm and less than or equal to 10nm. A microcrystal with a size greater than or equal to 1 nm and lessthan or equal to 10 nm is specifically referred to as nanocrystal (nc).An oxide semiconductor including nanocrystal is referred to as an nc-OS(nanocrystalline oxide semiconductor). In a TEM image of the nc-OS,since a clear grain boundary does not exist, segregation of an impurityis unlikely to occur. In the nc-OS, since a clear boundary does notexist, high density of defect states is unlikely to occur. In the nc-OS,since a clear grain boundary does not exist, a reduction in electronmobility is unlikely to occur.

In the nc-OS, a microscopic region (for example, a region with a sizegreater than or equal to 1 nm and less than or equal to 10 nm) has aperiodic atomic order occasionally. Further, in the nc-OS, crystal partsare not regularly-arranged. Thus, there is a case where periodic atomicorder is not observed macroscopically or a case where long-range orderin atomic arrangement is not observed. Accordingly, in some cases, thenc-OS cannot be distinguished from an amorphous oxide semiconductor,depending on an analysis method. When the nc-OS film is analyzed by anout-of-plane method with an XRD apparatus using an X-ray having a beamdiameter larger than that of a crystal part, a peak which showsalignment does not appear in some cases. Further, a halo pattern isshown in some cases in an electron diffraction pattern of the nc-OSobtained by using an electron beam having a diameter larger than that ofa crystal part (for example, a beam diameter of 20 nmϕ or more, or 50nmϕ or more). Spots are shown in some cases in a nanobeam electrondiffraction pattern of the nc-OS obtained by using an electron beamhaving a diameter smaller than or equal to that of a crystal part (forexample, a beam diameter of 10 nmϕ or less, or 5 nmϕ or less). In ananobeam electron diffraction pattern of the nc-OS, regions with highluminance in a circular pattern are shown in some cases. In a nanobeamelectron diffraction pattern of the nc-OS, a plurality of spots areshown in the region in some cases.

FIG. 10B shows an example of a nanobeam electron diffraction pattern ofa sample including nc-OS. Here, the sample is cut in the directionperpendicular to a surface where the nc-OS is formed and the thicknessthereof is reduced to about 40 nm. Further, an electron beam with adiameter of 1 nmϕ enters from the direction perpendicular to the cutsurface of the sample. FIG. 10B shows that spots are observed in thenanobeam electron diffraction pattern of the nc-OS.

Since the microscopic region in the nc-OS has a periodic atomic orderoccasionally, the nc-OS has lower density of defect states than theamorphous oxide semiconductor. Note that since crystal parts in thenc-OS are not regularly-arranged, the nc-OS has higher density of defectstates than the CAAC-OS.

Accordingly, the nc-OS has higher carrier density than the CAAC-OS insome cases. An oxide semiconductor with a high carrier density tends tohave a high electron mobility. Therefore, a transistor using the nc-OSfor a channel formation region has a high field-effect mobility in somecases. On the contrary, the nc-OS has higher density of defect statesthan the CAAC-OS and thus has higher density of trap states in somecases. Therefore, a transistor using the nc-OS for a channel formationregion has larger variation in electrical characteristics and lowerreliability than a transistor using the CAAC-OS for a channel formationregion. Note that the nc-OS can be obtained even when the amount ofimpurity contained therein is relatively large; thus, the nc-OS issometimes preferably used depending on the application. Further, thenc-OS may be formed by a deposition method such as a sputtering methodusing an AC power supply. The sputtering method using an AC power supplyallows a film to be formed with high uniformity over a large substrate,so that a semiconductor device including a transistor using the nc-OSfor a channel formation region can be manufactured with highproductivity.

An oxide semiconductor may include an amorphous part. Note that an oxidesemiconductor including an amorphous part is referred to as an amorphousoxide semiconductor. An amorphous oxide semiconductor has disorderedatomic arrangement and no crystal part. An amorphous oxide semiconductordoes not have a specific shape as in quartz and regularity in atomicarrangement.

In a TEM image of the amorphous oxide semiconductor, crystal partscannot be found in some cases.

When an amorphous oxide semiconductor is analyzed by an out-of-planemethod with an XRD apparatus, a peak which shows alignment does notappear in some cases. Further, a halo pattern is observed in an electrondiffraction pattern of the amorphous oxide semiconductor in some cases.In other cases, a halo pattern is observed instead of a spot in ananobeam electron diffraction pattern of the amorphous oxidesemiconductor.

In some cases, the amorphous oxide semiconductor can be formed byintroducing a high-concentration impurity such as hydrogen. Hence, theamorphous oxide semiconductor is an oxide semiconductor containing ahigh-concentration impurity.

When an oxide semiconductor contains a high-concentration impurity, adefect state such as an oxygen vacancy is formed in the oxidesemiconductor in some cases. This means that an amorphous oxidesemiconductor with a high-concentration impurity has a high density ofdefect states. In addition, since the amorphous oxide semiconductor haslow crystallinity, the density of defect states of the amorphous oxidesemiconductor is higher than that of the CAAC-OS or the nc-OS.

Thus, the amorphous oxide semiconductor has a much higher carrierdensity than the nc-OS in some cases. Therefore, a transistor using theamorphous oxide semiconductor for a channel formation region tends to benormally on, and sometimes can be preferably used for a transistor whichneeds to have such electrical characteristics. The amorphous oxidesemiconductor has a high density of defect states and thus has a highdensity of trap states in some cases. Therefore, a transistor using theamorphous oxide semiconductor for a channel formation region has largervariation in electrical characteristics and lower reliability than atransistor using the CAAC-OS or the nc-OS for a channel formation regionin some cases. Note that the amorphous oxide semiconductor can be formedby a deposition method in which a relatively large amount of impurity iscontained, and thus can be easily obtained and preferably used dependingon the application. For example, the amorphous oxide semiconductor maybe formed by a deposition method such as a spin coating method, asol-gel method, an immersion method, a spray method, a screen printingmethod, a contact printing method, an ink-jet printing method, a rollcoating method, or a mist CVD method. Hence, a semiconductor deviceincluding a transistor using the amorphous oxide semiconductor for achannel formation region can be manufactured with high productivity.

Note that the oxide semiconductor may be a mixed film including two ormore of a CAAC-OS, a polycrystalline oxide semiconductor, amicrocrystalline oxide semiconductor, and an amorphous oxidesemiconductor. The mixed film includes two or more of an amorphous oxidesemiconductor region, a microcrystalline oxide semiconductor region, apolycrystalline oxide semiconductor region, and a CAAC-OS region in somecases. The mixed film has a layered structure of two or more of anamorphous oxide semiconductor region, a microcrystalline oxidesemiconductor region, a polycrystalline oxide semiconductor region, anda CAAC-OS region in some cases.

An oxide semiconductor may include a single crystal, for example Notethat an oxide semiconductor including a single crystal is referred to asa single crystal oxide semiconductor.

The single crystal oxide semiconductor has, for example, a low impurityconcentration and a low density of defect states (a small number ofoxygen vacancies), and thus has a low carrier density. Therefore, atransistor using the single crystal oxide semiconductor for a channelformation region is unlikely to be normally on. Further, the singlecrystal oxide semiconductor has a low density of defect states and thushas a low density of trap states in some cases. Therefore, a transistorusing the single crystal oxide semiconductor for a channel formationregion has a small variation in electrical characteristics and a highreliability in some cases

The oxide semiconductor has a high density if including few defects insome cases. The oxide semiconductor has a high density if having a highcrystallinity in some cases. The oxide semiconductor has a high densityif including a low-concentration impurity such as hydrogen. Further, thedensity of a single crystal oxide semiconductor is higher than that of aCAAC-OS in some cases. Further, the density of a CAAC-OS is higher thanthat of a microcrystalline oxide semiconductor in some cases. Further,the density of a polycrystalline oxide semiconductor is higher than thatof a microcrystalline oxide semiconductor in some cases. Further, thedensity of a microcrystalline oxide semiconductor is higher than that ofan amorphous oxide semiconductor in some cases.

The CAAC-OS is preferably deposited under the following conditions.

By reducing the amount of impurities entering the CAAC-OS during thedeposition, the crystal state can be prevented from being broken by theimpurities. For example, the concentration of impurities (e.g.,hydrogen, water, carbon dioxide, or nitrogen) which exist in adeposition chamber may be reduced. Further, the concentration ofimpurities in a deposition gas may be reduced. Specifically, adeposition gas whose dew point is −80° C. or lower, preferably −100° C.or lower is used.

By increasing the heating temperature of the surface where the CAAC-OSis formed (e.g., the substrate heating temperature) during thedeposition, migration of a sputtered particle occurs after the sputteredparticle reaches the surface where the CAAC-OS is formed. Specifically,the temperature of the surface where the CAAC-OS is formed during thedeposition is higher than or equal to 100° C. and lower than or equal to740° C., preferably higher than or equal to 150° C. and lower than orequal to 500° C.

Furthermore, it is preferable that the proportion of oxygen in thedeposition gas be increased and the power be optimized in order toreduce plasma damage at the deposition. The proportion of oxygen in thedeposition gas is higher than or equal to 30 vol %, preferably 100 vol%.

An example of an In—Ga—Zn—O compound target as the sputtering target isdescribed below.

The In—Ga—Zn-based metal oxide target, which is polycrystalline, is madeby mixing InO_(X) powder, GaO_(Y) powder, and ZnO_(Z) powder in apredetermined molar ratio, applying pressure, and performing heattreatment at a temperature higher than or equal to 1000° C. and lowerthan or equal to 1500° C. The pressure treatment may be performed whilecooling is performed or may be performed while heating is performed.Note that X, Y, and Z are each a given positive number. Here, thepredetermined molar ratio of InO_(X) powder to GaO_(Y) powder andZnO_(Z) powder is, for example, 2:2:1, 8:4:3, 3:1:1, 1:1:1, 4:2:3, or3:1:2. The kinds of powder and the molar ratio for mixing powder may bedetermined as appropriate depending on the sputtering target that is tobe formed.

Table 1 shows the comparison between oxide semiconductors (OS) andsilicon (Si) in a crystal state.

TABLE 1 Continuous Single Amorphous Microcrystal Polycrystal crystalcrystal OS a-OS nc-OS Polycrystalline CAAC-OS Single a-OS:H μc-OS OScrystal OS Nanobeam Halo Ring + Spot Spot Spot electron Spot diffractionCrystal part — nm-μm Discontinuous Continuously — connected DOS HighSlightly low — Low Extremely low Density Low Medium — High — Si a-Sinc-Si Polycrystalline CG silicon Single a-Si:H μc-Si Si crystal Si

Examples of the crystal state of the oxide semiconductor include, asshown in Table 1, an amorphous oxide semiconductor (a-OS, a-OS: H), amicrocrystalline oxide semiconductor (nc-OS, pc-OS), a polycrystallineoxide semiconductor (polycrystalline OS), a continuous crystal oxidesemiconductor (CAAC-OS), and a single crystal oxide semiconductor(single crystal OS). Examples of the crystal state of silicon include,as shown in Table 1, amorphous silicon (a-Si, a-Si: H), microcrystallinesilicon (nc-Si, μc-Si), polycrystalline silicon (polycrystalline Si),continuous crystal silicon (CG (continuous grain) silicon), and singlecrystal silicon (single crystal Si).

The oxide semiconductors in the above crystal states are subjected toelectron diffraction using an electron beam with a diameter reduced to10 nmϕ or less (nanobeam electron diffraction). Then, the followingelectron diffraction patterns (nanobeam electron diffraction patterns)are observed. A halo pattern (also called a halo ring or a halo) isobserved in the amorphous oxide semiconductor. A spot or/and a ringpattern is/are observed in the microcrystalline oxide semiconductor. Aspot is observed in the polycrystalline oxide semiconductor. A spot isobserved in the continuous crystal oxide semiconductor. A spot isobserved in the single crystal oxide semiconductor.

The nanobeam electron diffraction pattern shows that themicrocrystalline oxide semiconductor includes a crystal part with adiameter of nanometers (nm) to micrometers (μm). The polycrystallineoxide semiconductor is found to have a grain boundary between crystalparts, that is, a discontinuous boundary. The continuous crystal oxidesemiconductor is found to have no boundary between crystal parts, thatis, a continuous boundary.

The density of the oxide semiconductor in each crystal state is asfollows. The amorphous oxide semiconductor has a low density. Themicrocrystalline oxide semiconductor has a medium density. Thecontinuous crystal oxide semiconductor has a high density. In otherwords, the density of the continuous crystal oxide semiconductor ishigher than that of the microcrystalline oxide semiconductor, and thedensity of the microcrystalline oxide semiconductor is higher than thatof the amorphous oxide semiconductor.

Density of states (DOS) in the oxide semiconductor in each crystal stateis as follows. The amorphous oxide semiconductor has a high DOS. Themicrocrystalline oxide semiconductor has a rather low DOS. Thecontinuous crystal oxide semiconductor has a low DOS. The single crystaloxide semiconductor has an extremely low DOS. In other words, the DOS ofthe single crystal oxide semiconductor is lower than that of thecontinuous crystal oxide semiconductor, the DOS of the continuouscrystal oxide semiconductor is lower than that of the microcrystallineoxide semiconductor, and the DOS of the microcrystalline oxidesemiconductor is lower than that of the amorphous oxide semiconductor.

The oxide semiconductor film may include a plurality of oxidesemiconductor films stacked. For example, as in a transistor illustratedin FIG. 9A, a first oxide semiconductor film 188 a and a second oxidesemiconductor film 188 b may be stacked to be used as the semiconductorfilm. The first oxide semiconductor film 188 a and the second oxidesemiconductor film 188 b may include metal oxides having differentatomic ratios. For example, one of the oxide semiconductor films mayinclude one of an oxide containing two kinds of metals, an oxidecontaining three kinds of metals, and an oxide containing four kinds ofmetals, and the other of the oxide semiconductor films may includeanother one of the oxide containing two kinds of metals, the oxidecontaining three kinds of metals, and the oxide containing four kinds ofmetals.

Alternatively, the first oxide semiconductor film 188 a and the secondoxide semiconductor film 188 b may include the same constituent elementswith different atomic ratios. For example, one of the oxidesemiconductor films may contain In, Ga, and Zn at an atomic ratio of3:1:2, and the other of the oxide semiconductor films may contain In,Ga, and Zn at an atomic ratio of 1:1:1. Alternatively, one of the oxidesemiconductor films may contain In, Ga, and Zn at an atomic ratio of2:1:3, and the other of the oxide semiconductor films may contain In,Ga, and Zn at an atomic ratio of 1:3:2. Alternatively, one of the oxidesemiconductor films may contain In, Ga, and Zn at an atomic ratio of1:1:1, and the other of the oxide semiconductor films may contain In,Ga, and Zn at an atomic ratio of 1:3:2. Alternatively, one of the oxidesemiconductor films may contain In, Ga, and Zn at an atomic ratio of1:1:1, and the other of the oxide semiconductor films may contain In,Ga, and Zn at an atomic ratio of 1:6:4. Alternatively, one of the oxidesemiconductor films may contain In, Ga, and Zn at an atomic ratio of1:1:1, and the other of the oxide semiconductor films may contain In,Ga, and Zn at an atomic ratio of 1:9:6. Note that the proportion of eachatom in the atomic ratio of the oxide semiconductor film varies within arange of ±20% as an error.

In the above, one of the first oxide semiconductor film and the secondoxide semiconductor film, which is closer to the gate electrode (theoxide semiconductor film on the channel side), has an atomic ratio of In≥Ga; and the other oxide semiconductor film, which is farther from thegate electrode (the oxide semiconductor film on the back channel side),has an atomic ratio of In<Ga. In that case, a transistor with a highfield-effect mobility can be manufactured. On the other hand, when theoxide semiconductor film on the channel side has an atomic ratio ofIn<Ga and the oxide semiconductor film on the back channel side has anatomic ratio of In ≥Ga, it is possible to reduce the amount of change inthe threshold voltage of a transistor due to change over time or areliability test.

Further alternatively, the semiconductor film of the transistor may havea three-layer structure of a first oxide semiconductor film, a secondoxide semiconductor film, and a third oxide semiconductor film In thatcase, the first to third oxide semiconductor films may include the sameconstituent elements with different atomic ratios. A transistorincluding a three-layer semiconductor film will be described withreference to FIG. 9B.

The transistor illustrated in FIG. 9B includes a first oxidesemiconductor film 199 a, a second oxide semiconductor film 199 b, and athird oxide semiconductor film 199 c which are stacked in this orderfrom the gate insulating film 127 side. The first oxide semiconductorfilm 199 a and the third oxide semiconductor film 199 c are formed usinga material represented by InM_(1x)Zn_(y)O_(z) (x≥1, y>1, z>0, M₁=Ga, Hf,or the like). Note that in the case where each of the first oxidesemiconductor film 199 a and the third oxide semiconductor film 199 ccontains Ga, a material containing a large proportion of Ga,specifically, a material which can be represented by InM_(1x)Zn_(y)O_(z)where x is larger than 10 is unsuitable because powder might begenerated in deposition.

The second oxide semiconductor film 199 b is formed using a materialwhich can be represented by InM_(2x)Zn_(y)O_(z) (x≥1, y≥x, z>0, M₂=Ga,Sn, or the like).

Materials of the first to third oxide semiconductor films 199 a to 199 care selected as appropriate so as to form a well-shaped structure inwhich the bottom of the conduction band in the second oxidesemiconductor film 199 b is deeper from the vacuum level than thebottoms of the conduction band in the first and third oxidesemiconductor films 199 a and 199 c.

Note that silicon and carbon, which are Group 14 elements, sometimecontribute to the formation of donor levels in an oxide semiconductorfilm Therefore, silicon or carbon contained in the oxide semiconductorfilm makes an n-type oxide semiconductor film. Thus, the oxidesemiconductor film is preferably formed so as to have a region where theconcentration of each of silicon and carbon is less than or equal to3×10¹⁸/cm³, preferably less than or equal to 3×10¹⁷/cm³. It isparticularly preferable that the second oxide semiconductor film 199 bserving as a carrier path be sandwiched between or surrounded by thefirst and third oxide semiconductor films 199 a and 199 c so that alarge number of Group 14 elements do not enter the second oxidesemiconductor film 199 b. That is to say, the first and third oxidesemiconductor films 199 a and 199 c can also be called barrier filmswhich prevent Group 14 elements such as silicon and carbon from enteringthe second oxide semiconductor film 199 b.

For example, the first oxide semiconductor film 199 a and the thirdoxide semiconductor film 199 c may each have an atomic ratio ofIn:Ga:Zn=1:3:2, 1:6:4, or 1:9:6; the second oxide semiconductor film 199b may have an atomic ratio of In:Ga:Zn=1:1:1 or 3:1:2.

Alternatively, the first oxide semiconductor film 199 a may have anatomic ratio of In:Ga:Zn=1:3:2, the second oxide semiconductor film 199b may have an atomic ratio of In:Ga:Zn=1:1:1 or 3:1:2, and the thirdoxide semiconductor film 199 c may have an atomic ratio ofIn:Ga:Zn=1:6:4 or 1:9:6.

Since the first to third oxide semiconductor films 199 a to 199 cinclude the same constituent elements, the second oxide semiconductorfilm 199 b has few defect states (trap levels) at the interface with thefirst oxide semiconductor film 199 a. Specifically, the defect states(trap levels) are fewer than those at the interface between the gateinsulating film 127 and the first oxide semiconductor film 199 a. Forthis reason, when the oxide semiconductor films are stacked in the abovemanner, the amount of change in the threshold voltage of a transistordue to a change over time or a reliability test can be reduced.

Further, materials of the first to third oxide semiconductor films 199 ato 199 c are selected as appropriate so as to form a well-shapedstructure in which the bottom of the conduction band in the second oxidesemiconductor film 199 b is deeper from the vacuum level than thebottoms of the conduction band in the first and third oxidesemiconductor films 199 a and 199 c. As a result, the field-effectmobility of the transistor can be increased and the amount of change inthe threshold voltage of the transistor due to change over time or areliability test can be reduced.

Further, the first to third oxide semiconductor films 199 a to 199 c maybe formed using oxide semiconductors having different crystallinities.That is, the first to third oxide semiconductor films may be formedusing a combination of any of a single crystal oxide semiconductor, apolycrystalline oxide semiconductor, a microcrystalline(nanocrystalline) oxide semiconductor, an amorphous oxide semiconductor,and a CAAC-OS. By using an amorphous oxide semiconductor for any one ofthe first to third oxide semiconductor films 199 a to 199 c, it ispossible to relieve the internal stress or external stress of the oxidesemiconductor film, reduce a variation in the characteristics of atransistor, and reduce a variation in the threshold voltage of thetransistor due to a change over time or a reliability test.

At least the second oxide semiconductor film 199 b, which can serve as achannel formation region, is preferably a CAAC-OS film.

When the oxide semiconductor film is in contact with a conductivematerial which is easily bonded to oxygen (e.g., a metal used for thesource electrode or the drain electrode), oxygen in the oxidesemiconductor film is diffused into the conductive material which iseasily bonded to oxygen. Such phenomenon occurs more frequently as thetemperature is raised. Some heating steps are included in themanufacturing process of the transistor; thus, owing to the phenomenon,oxygen vacancies are generated in and around the region of the oxidesemiconductor layer which is in contact with the source electrode or thedrain electrode, so that the region becomes an n-type region.Accordingly, the n-type region can serve as a source or drain of thetransistor.

The above n-type region is illustrated in FIGS. 9A and 9B. A boundary135 indicated by a dotted line in the semiconductor film is the boundarybetween an intrinsic semiconductor region and the n-type semiconductorregion. In the oxide semiconductor, a region near and in contact withthe source electrode or the drain electrode becomes the n-type region.The boundary 135 is schematically illustrated here, but actually theboundary is not clearly seen in some cases. The position of the boundary135 is also different in some cases.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

Embodiment 3

Described in this embodiment are the electron diffraction pattern andlocalized states of a nanocrystalline oxide semiconductor film that canbe used in one embodiment of the present invention.

When a nanocrystalline oxide semiconductor film is observed withelectron diffraction (nanobeam electron diffraction) with a beamdiameter of 10 nmϕ or less, spots without directionality are found inthe electron diffraction pattern thereof. This electron diffractionpattern of the nanocrystalline oxide semiconductor film is neither ahalo pattern indicating an amorphous state nor a pattern with regularspots indicating a crystal state in which crystals are aligned in aspecific surface.

FIG. 13A shows a cross-sectional transmission electron microscopy (TEM)image of the nanocrystalline oxide semiconductor film. FIGS. 13B, 13C,and 13D show electron diffraction patterns on points 1, 2, and 3 in FIG.13A, respectively, which are observed by nanobeam electron diffraction.

In FIGS. 13A to 13D, as an example of the nanocrystalline oxidesemiconductor film, a 50-nm-thick In—Ga—Zn-based oxide film was formedover a quartz glass substrate. The nanocrystalline oxide semiconductorfilm was formed under the following conditions: an oxide targetcontaining In, Ga, and Zn at an atomic ratio of 1:1:1 was used; anoxygen atmosphere (flow rate of 45 sccm) was used; the pressure was 0.4Pa; a direct-current (DC) power of 0.5 kW was applied; and the substratetemperature was room temperature. Then, the width of the formednanocrystalline oxide semiconductor film was reduced to less than orequal to 100 nm (e.g., 40 nm±10 nm) and a cross-sectional TEM image andnanobeam electron diffraction patterns were observed.

FIG. 13A shows the cross-sectional TEM image of the nanocrystallineoxide semiconductor film which was taken with a transmission electronmicroscope (“H-9000NAR” manufactured by Hitachi High-TechnologiesCorporation) at an acceleration voltage of 300 kV and a magnification of2000000 times. FIGS. 13B to 13D show electron diffraction patternsobtained by nanobeam electron diffraction with a beam diameter of about1 nmϕ, which was observed with a transmission electron microscope(“HF-2000” manufactured by Hitachi High-Technologies Corporation) at anacceleration voltage of 200 kV. Note that the measurement area of thenanobeam electron diffraction with a beam diameter of about 1 nmϕ wasgreater than or equal to 5 nmϕ and less than or equal to 10 nmϕ.

As shown in FIG. 13B, when the nanocrystalline oxide semiconductor filmis observed with the nanobeam electron diffraction, an electrondiffraction pattern including circumferentially arranged spots (brightpoints) is obtained. In other words, in the nanocrystalline oxidesemiconductor film, a plurality of circumferentially (concentrically)distributed spots are observed, or a plurality of circumferentiallydistributed spots form a plurality of concentric circles.

A plurality of circumferentially distributed spots are observed not onlyin FIG. 13B, but also in FIG. 13C which shows the center of thenanocrystalline oxide semiconductor film in the thickness direction andin FIG. 13D which shows the vicinity of the interface with the quartzglass substrate. In FIG. 13C, the distance from a main spot to thecircumferentially distributed spots is in a range from 3.88/nm to4.93/nm, or from 0.203 nm to 0.257 nm when converted into interplanarspacing.

The nanobeam electron diffraction patterns in FIGS. 13A to 13D show thatthe nanocrystalline oxide semiconductor film includes a plurality ofcrystal parts with irregular surface orientations and different sizes.

FIG. 14A shows a plane TEM image of the nanocrystalline oxidesemiconductor film FIG. 14B shows an electron diffraction pattern of aregion surrounded by a circle in FIG. 14A, which is observed byselected-area electron diffraction.

In FIGS. 14A and 14B, as an example of the nanocrystalline oxidesemiconductor film, a 30-nm-thick In—Ga—Zn-based oxide film was formedover a quartz glass substrate. The nanocrystalline oxide semiconductorfilm in FIGS. 14A and 14B was formed under the following conditions: anoxide target containing In, Ga, and Zn at an atomic ratio of 1:1:1 wasused; an oxygen atmosphere (flow rate of 45 sccm) was used; the pressurewas 0.4 Pa; a direct-current (DC) power of 0.5 kW was applied; and thesubstrate temperature was room temperature. Then, the sample wasthinned, and a plane TEM image and a selected-area electron diffractionpattern of the nanocrystalline oxide semiconductor film were observed.

FIG. 14A shows the plane TEM image of the nanocrystalline oxidesemiconductor film which was taken with a transmission electronmicroscope (“H-9000NAR” manufactured by Hitachi High-TechnologiesCorporation) at an acceleration voltage of 300 kV and at a magnificationof 500000 times. FIG. 14B shows the electron diffraction patternobtained by electron diffraction with a selected area of 300 nmϕ. Notethat the measurement area in FIG. 14B is greater than or equal to 300nmϕ in consideration of electron beam expansion (about severalnanometers).

As shown in FIG. 14B, when the nanocrystalline oxide semiconductor filmis observed by selected-area electron diffraction the measurement areaof which is wider than that of the nanobeam electron diffraction, theplurality of spots observed by nanobeam electron diffraction are notobserved in the electron diffraction pattern and a halo pattern isobserved.

FIGS. 15A to 15C conceptually show diffraction intensity distribution inthe electron diffraction patterns in FIGS. 13A to 13D and FIGS. 14A and14B. FIG. 15A shows a conceptual diagram of diffraction intensitydistribution in the nanobeam electron diffraction patterns in FIGS. 13Bto 13D. FIG. 15B shows a conceptual diagram of diffraction intensitydistribution in the selected-area electron diffraction pattern in FIG.14B. FIG. 15C shows a conceptual diagram of diffraction intensitydistribution in an electron diffraction pattern of a single crystalstructure or a polycrystalline structure.

In each of FIGS. 15A to 15C, the vertical axis represents electrondiffraction intensity (arbitrary unit) indicating distribution of spotsor the like and the horizontal axis represents the distance from a mainspot.

In the single crystal structure or the polycrystalline structure in FIG.15C, peaks are observed at a specific distance from the main spot, whichis based on the interplanar spacing (d value) between planes with whichcrystal parts are aligned.

On the other hand, as shown in FIGS. 13B to 13D, a circumferentialregion formed by the plurality of spots observed in the nanobeamelectron diffraction pattern of the nanocrystalline oxide semiconductorfilm has a relatively large width. Thus, FIG. 15A shows discretedistribution. Further, in the nanobeam electron diffraction pattern, aregion with high luminance formed by spots which are not clear isobserved in a region between concentric circles.

As for the selected-area electron diffraction pattern of thenanocrystalline oxide semiconductor film, the electron diffractionintensity distribution is continuous as shown in FIG. 15B. Since FIG.15B can approximate to the result obtained by widely observing theelectron diffraction intensity distribution shown in FIG. 15A, thecontinuous intensity distribution can be considered to result from theoverlapping and connection of the plurality of spots.

FIGS. 15A to 15C indicate that the nanocrystalline oxide semiconductorfilm includes a plurality of crystal parts with irregular surfaceorientations and different sizes, and the crystal parts are so minutethat spots are not observed in the selected-area electron diffractionpatterns.

In FIGS. 13A to 13D where the plurality of spots are observed, the widthof the nanocrystalline oxide semiconductor film is reduced to less thanor equal to 50 nm. Further, since the diameter of an electron beam isreduced to 1 nmϕ or less, the measurement area is greater than or equalto 5 nm and less than or equal to 10 nm. It is thus assumed that thecrystal part included in the nanocrystalline oxide semiconductor filmhas a size of 50 nm or less, for example, 10 nm or less, or 5 nm orless.

FIG. 16 shows a nanobeam electron diffraction pattern of a quartz glasssubstrate. The measurement conditions in FIG. 16 were similar to thosein FIGS. 13B to 13D.

As shown in FIG. 16, in the quartz glass substrate having an amorphousstructure, specific spots are not included and a halo pattern in whichluminance is gradually changed from a main spot is observed. This meansthat a plurality of circumferentially distributed spots like thoseobserved in the nanocrystalline oxide semiconductor film are notobserved in a film having an amorphous structure even when electrondiffraction is performed on a minute region. This indicates that theplurality of circumferentially distributed spots observed in FIGS. 13Bto 13D are unique to the nanocrystalline oxide semiconductor film.

FIG. 17 shows an electron diffraction pattern that is observed after thepoint 2 in FIG. 13A is irradiated for one minute with an electron beamwhose diameter was reduced to about 1 nmϕ.

As in the electron diffraction pattern shown in FIG. 13C, a plurality ofcircumferentially distributed spots are observed in the electrondiffraction pattern shown in FIG. 17, and there is no significantdifference between the measurement results in FIG. 13C and FIG. 17. Thismeans that the crystal part observed in the electron diffraction patternin FIG. 13C exists at the time of the deposition of the nanocrystallineoxide semiconductor film, and does not result from the irradiation withan electron beam with reduced diameter.

FIGS. 18A and 18B show enlarged views of part of the cross-sectional TEMimage of FIG. 13A. FIG. 18A shows a cross-sectional TEM image of thevicinity of the point 1 (a surface of the nanocrystalline oxidesemiconductor film) in FIG. 13A, which is observed at a magnification of8000000 times. FIG. 18B shows a cross-sectional TEM image of thevicinity of the point 2 (the center of the nanocrystalline oxidesemiconductor film in the thickness direction) in FIG. 13A, which isobserved at a magnification of 8000000 times.

In the cross-sectional TEM images of the nanocrystalline oxidesemiconductor film in FIGS. 18A and 18B, a crystal structure is notclearly observed.

The sample used in FIGS. 13A to 13D and FIGS. 14A and 14B, in which thenanocrystalline oxide semiconductor film of this embodiment is depositedover the quartz glass substrate, was analyzed by X-ray diffraction(XRD). FIG. 19 shows an XRD spectrum measured by an out-of-plane method.

In FIG. 19, the vertical axis represents the X-ray diffraction intensity(arbitrary unit) and the horizontal axis represents the diffractionangle 2θ (degree). Note that the XRD spectra were measured with an X-raydiffractometer, D8 ADVANCE manufactured by Bruker AXS.

As shown in FIG. 19, a peak corresponding to quartz appears at around2θ=20° to 23°; however, a peak corresponding to the crystal partincluded in the nanocrystalline oxide semiconductor film cannot befound.

The results of FIGS. 18A and 18B and FIG. 19 indicate that the crystalpart in the nanocrystalline oxide semiconductor film is a minute crystalpart.

As described above, in the nanocrystalline oxide semiconductor film ofthis embodiment, a peak showing alignment is not found by X-raydiffraction analysis with a wide measurement range, while a halo patternis observed in a selected-area electron diffraction pattern with alimited measurement range. This indicates that the nanocrystalline oxidesemiconductor film of this embodiment is macroscopically equivalent to afilm having disordered atomic arrangement. However, when thenanocrystalline oxide semiconductor film is observed by nanobeamelectron diffraction with a sufficiently small beam diameter (e.g., lessthan or equal to 10 nmϕ), spots (bright points) can be found in theobtained nanobeam electron diffraction pattern. It is thus assumed thatthe nanocrystalline oxide semiconductor film of this embodiment isformed by the aggregation of minute crystal parts (e.g., a particlediameter of 10 nm or less, 5 nm or less, or 3 nm or less) with randomsurface orientations. A nanocrystal region including minute crystalparts is included in all the area of the nanocrystalline oxidesemiconductor film in the thickness direction.

Here, localized states of the nanocrystalline oxide semiconductor filmwill be described. Description is made on the measurement results of thenanocrystalline oxide semiconductor film with a constant photocurrentmethod (CPM).

First, a structure of a measurement sample will be described.

The measurement sample includes an oxide semiconductor film over a glasssubstrate, a pair of electrodes in contact with the oxide semiconductorfilm, and an insulating film covering the oxide semiconductor film andthe pair of electrodes.

Next, description will be made on a method for forming the oxidesemiconductor film included in the measurement sample.

A first oxide semiconductor film was formed by a sputtering method underthe following conditions: an In—Ga—Zn-oxide target (In:Ga:Zn=1:1:1[atomic ratio]) was used; argon with a flow rate of 30 sccm and oxygenwith a flow rate of 15 sccm were used as a deposition gas; the pressurewas 0.4 Pa; the substrate temperature was room temperature; and adirect-current power of 0.5 kW was applied. Note that the first oxidesemiconductor film is a nanocrystalline oxide semiconductor film.

The first oxide semiconductor film was heated for one hour in a nitrogenatmosphere at 450° C. and then heated for one hour in an oxygenatmosphere at 450° C., whereby hydrogen was removed from the first oxidesemiconductor film and oxygen was supplied to the first oxidesemiconductor film; thus, a second oxide semiconductor film wasobtained. Note that the second oxide semiconductor film is ananocrystalline oxide semiconductor film.

Next, CPM measurement was performed on a sample including the firstoxide semiconductor film and a sample including the second oxidesemiconductor film. Specifically, the amount of light entering a surfaceof the sample between terminals is adjusted so that a photocurrent valueis kept constant in the state where voltage is applied between a pair ofelectrodes provided in contact with the oxide semiconductor film, andthen an absorption coefficient is derived from the amount of theirradiation light in an intended wavelength range.

The absorption coefficient shown in each of FIGS. 11A and 11B wasobtained by removing an absorption coefficient due to the band tail froman absorption coefficient obtained by CPM measurement of the samples.That is, FIGS. 11A and 11B show the absorption coefficient due todefects. In FIGS. 11A and 11B, the horizontal axis indicates theabsorption coefficient, and the vertical axis indicates the photonenergy. On the vertical axis in FIGS. 11A and 11B, the bottom of theconduction band of the oxide semiconductor film is set to 0 eV, and thetop of the valence band is set to 3.15 eV. Each curve in FIGS. 11A and11B represents the relation between the absorption coefficient andphoton energy, which corresponds to defect states.

FIG. 11A shows a measurement result of the sample including the firstoxide semiconductor film, and the absorption coefficient indicatingdefect states was 5.28×10⁻¹ cm⁻¹. FIG. 11B shows a measurement result ofthe sample including the second oxide semiconductor film, and theabsorption coefficient indicating defect states was 1.75×10⁻² cm⁻¹.

The above shows that defects in the oxide semiconductor film can bereduced by heat treatment.

The film densities of the first oxide semiconductor film and the secondoxide semiconductor film were measured by X-ray reflectometry (XRR). Thefirst oxide semiconductor film has a film density of 5.9 g/cm³ and thesecond oxide semiconductor film has a film density of 6.1 g/cm³.

This indicates that the film density of the oxide semiconductor film canbe increased by heat treatment.

In other words, the oxide semiconductor film with higher film densityincludes fewer defects.

This embodiment can be combined with any of the other embodiments shownin this specification.

Embodiment 4

Described in this embodiment are electron diffraction patterns andlocalized states of a CAAC-OS film which can be used for one embodimentof the present invention.

The CAAC-OS film used in this embodiment is an In—Ga—Zn-based oxide filmwhich is formed by a sputtering method using a deposition gas containingoxygen and a target of an In—Ga—Zn oxide (having an atomic ratio ofIn:Ga:Zn=1:1:1). Embodiments 1 and 2 can be referred to for the detailsof a manufacturing method and the like of the CAAC-OS film.

FIG. 20 shows a cross-sectional transmission electron microscopy (TEM)image of the CAAC-OS film FIGS. 21A to 21D show electron diffractionpatterns which were obtained by measurement of points 1 to 4 in FIG. 20using electron diffraction.

The cross-sectional TEM image shown in FIG. 20 was taken with atransmission electron microscope (“H-9000NAR” manufactured by HitachiHigh-Technologies Corporation) at an acceleration voltage of 300 kV andat a magnification of 2,000,000 times. The electron diffraction patternsshown in FIGS. 21A to 21D were obtained with a transmission electronmicroscope (“HF-2000” manufactured by Hitachi High-TechnologiesCorporation) at an acceleration voltage of 200 kV and beam diameters ofabout 1 nmϕ or about 50 nmϕ. Note that electron diffraction with a beamdiameter of 10 nmϕ or less is particularly referred to as nanobeamelectron diffraction in some cases. Further, a measurement area of theelectron diffraction with a beam diameter of about 1 nmϕ is greater thanor equal to 5 nmϕ and less than or equal to 10 nmϕ.

Electron diffraction patterns on point 1 (the surface side of the film),point 2 (the center of the film), and point 3 (the base side of thefilm) shown in FIG. 20 correspond to FIGS. 21A, 21B, and 21C,respectively and are obtained with an electron-beam diameter of about 1nmϕ. An electron diffraction pattern on point 4 (the whole film) shownin FIG. 20 corresponds to FIG. 21D and is obtained with an electron-beamdiameter of about 50 nmϕ.

A pattern formed by spots (bright points) is observed in each of theelectron diffraction patterns of point 1 (on the surface side of thefilm) and point 2 (the center of the film), and a slightly brokenpattern is observed in point 3 (on the base side of the film). Thisindicates that the crystal state varies in the thickness direction inthe CAAC-OS film Note that a pattern formed by spots (bright points) isobserved in point 4 (the whole film), which indicates that the wholefilm is a CAAC-OS film or a film including a CAAC-OS film.

FIG. 22 shows an enlarged view of a portion in the vicinity of point 1(on the surface side of the film) in FIG. 20. In FIG. 22, a clearlattice image which shows alignment of the CAAC-OS film extends to theinterface with an SiON film that is an interlayer insulating film.

FIGS. 23A and 23B show a cross-sectional TEM photograph and an X-raydiffraction spectrum of a CAAC-OS film different from the CAAC-OS filmused for the cross-sectional TEM image of FIG. 20. A CAAC-OS film mayhave a variety of forms, and Peak A which indicates a crystal componentappears around 2θ=31° as shown in FIG. 23B, but the peak does not appearclearly in some cases.

FIGS. 24A to 24D show results of electron diffraction in regions in theCAAC-OS film with electron-beam diameters of 1 nmϕ, 20 nmϕ, 50 nmϕ, and70 nmϕ. The regions are indicated by concentric circles in FIG. 23A. Inthe case of an electron-beam diameter of 1 nmϕ, a pattern formed byclear spots (bright points) can be observed as in FIGS. 21A and 21B. Asthe electron-beam diameter is increased, the spots (bright points)become unclear but a diffraction pattern can be observed; therefore, thewhole film is a CAAC-OS film or a film including a CAAC-OS film.

FIGS. 25A and 25B show a cross-sectional TEM image and an X-raydiffraction spectrum of the CAAC-OS film used for the cross-sectionalTEM observation in FIG. 23A, which are obtained after annealing at 450°C.

FIGS. 26A to 26D show results of electron diffraction in regions in theCAAC-OS film with electron-beam diameters of 1 nmϕ, 20 nmϕ, 50 nmϕ, and70 nmϕ. The regions are indicated by concentric circles in FIG. 25A. Inthe case of an electron-beam diameter of 1 nmϕ, a pattern formed byclear spots (bright points) can be observed as in the results shown inFIGS. 24A to 24D. As the electron-beam diameter is increased, the spots(bright points) become unclear but a diffraction pattern can beobserved; therefore, the whole film is a CAAC-OS film or a filmincluding a CAAC-OS film.

FIGS. 27A and 27B show a cross-sectional TEM image and an X-raydiffraction spectrum of a CAAC-OS film different from the CAAC-OS filmused for the cross-sectional TEM image of FIG. 20 and thecross-sectional TEM observation of FIG. 23A. The CAAC-OS film has avariety of forms, and as shown in FIG. 27B, as well as Peak A indicatinga crystal component around 2θ=31°, Peak B which is derived from a spinelcrystal structure appears in some cases.

FIGS. 28A to 28D show results of electron diffraction in regions in theCAAC-OS film with electron-beam diameters of 1 nmϕ, 20 nmϕ, 50 nmϕ, and90 nmϕ. The regions are indicated by concentric circles in FIG. 27A. Inthe case of an electron-beam diameter of 1 nmϕ, a pattern formed byclear spots (bright points) can be observed. As the electron-beamdiameter is increased, the spots (bright points) become unclear but adiffraction pattern can be observed. Further, in the case of a beamdiameter of 90 nmϕ, clearer spots (bright points) can be observed.Accordingly, the whole film is a CAAC-OS film or a film including aCAAC-OS film.

Here, localized states of the CAAC-OS film will be described.Description is made on the measurement results of the CAAC-OS film witha constant photocurrent method (CPM).

First, a structure of a sample subjected to CPM measurement will bedescribed.

The measurement sample includes an oxide semiconductor film over a glasssubstrate, a pair of electrodes in contact with the oxide semiconductorfilm, and an insulating film covering the oxide semiconductor film andthe pair of electrodes.

Next, description will be made on a method for forming the oxidesemiconductor film included in the measurement sample.

An oxide semiconductor film was formed by a sputtering method under thefollowing conditions: an In—Ga—Zn-oxide target (In:Ga:Zn=1:1:1 [atomicratio]) was used; argon with a flow rate of 30 sccm and oxygen with aflow rate of 15 sccm were used as a deposition gas; the pressure was 0.4Pa; the substrate temperature was 400° C.; and a direct-current power of0.5 kW was applied. Then, the oxide semiconductor film was heated forone hour in a nitrogen atmosphere at 450° C. and then heated for onehour in an oxygen atmosphere at 450° C., whereby hydrogen was removedfrom the oxide semiconductor film and oxygen was supplied to the oxidesemiconductor film. Note that this oxide semiconductor film is a CAAC-OSfilm.

Next, CPM measurement was performed on the sample including the oxidesemiconductor film. Specifically, the amount of light entering a surfaceof the sample between terminals is adjusted so that a photocurrent valueis kept constant in the state where voltage is applied between a pair ofelectrodes provided in contact with the oxide semiconductor film, andthen an absorption coefficient is derived from the amount of theirradiation light in an intended wavelength range.

FIG. 12 shows the absorption coefficient that was obtained by removingan absorption coefficient due to the band tail from an absorptioncoefficient obtained by CPM measurement of the sample. That is, FIG. 12shows the absorption coefficient due to defects. In FIG. 12, thehorizontal axis indicates the absorption coefficient, and the verticalaxis indicates the photon energy. On the vertical axis in FIG. 12, thebottom of the conduction band of the oxide semiconductor film is set to0 eV, and the top of the valence band is set to 3.15 eV. The curve inFIG. 12 represents the relation between the absorption coefficient andphoton energy, which corresponds to defect states.

In FIG. 12, the absorption coefficient indicating defect states was5.86×10⁻⁴ cm⁻¹. That is to say, the CAAC-OS film has an absorptioncoefficient due to defect states of less than 1×10⁻³/cm, preferably lessthan 1×10⁻⁴/cm, namely, has a low density of defect states.

The film density of the oxide semiconductor film was measured by X-rayreflectometry (XRR). The oxide semiconductor film has a film density of6.3 g/cm³. That is, the CAAC-OS film has a high film density.

This embodiment can be combined with any of the other embodiments shownin this specification.

Embodiment 5

A semiconductor device (display device) having a display function can befabricated using a transistor and a capacitor examples of which aredescribed in the above embodiments. Moreover, some or all of the drivercircuits which include the transistor can be formed over a substratewhere the pixel portion is formed, whereby a system-on-panel can beobtained. In this embodiment, an example of a display device using thetransistor examples of which are shown in the above embodiments will bedescribed with reference to FIGS. 29A to 29C, FIG. 30, and FIGS. 31A to31C. FIG. 30 is a cross-sectional view taken along dashed-dotted lineM-N in FIG. 29B. Note that FIG. 30 illustrates only part of thestructure of the pixel portion.

In FIG. 29A, a sealant 905 is provided so as to surround a pixel portion902 over a first substrate 901, and the pixel portion 902 is sealed witha second substrate 906. In FIG. 29A, a second driver circuit 903 and afirst driver circuit 904 each are formed using a single crystalsemiconductor or a polycrystalline semiconductor over a substrateprepared separately, and mounted in a region different from the regionsurrounded by the sealant 905 over the first substrate 901. Further,various signals and potentials are supplied to the second driver circuit903, the first driver circuit 904, and the pixel portion 902 fromflexible printed circuits (FPCs) 918 a and 918 b.

The first driver circuit 904 functions as a scan line drive circuit, andthe second driver circuit 903 functions as a signal line driver circuit.

In FIGS. 29B and 29C, the sealant 905 is provided so as to surround thepixel portion 902 and the first driver circuit 904 which are providedover the first substrate 901. The second substrate 906 is provided overthe pixel portion 902 and the first driver circuit 904. Thus, the pixelportion 902 and the first driver circuit 904 are sealed together with adisplay element by the first substrate 901, the sealant 905, and thesecond substrate 906. In FIGS. 29B and 29C, the second driver circuit903 which is formed using a single crystal semiconductor or apolycrystalline semiconductor is provided over a substrate separatelyprepared and is mounted in a region different from the region surroundedby the sealant 905 over the first substrate 901. In FIGS. 29B and 29C,various signals and potentials are supplied to the second driver circuit903, the first driver circuit 904, and the pixel portion 902 from theFPC 918.

Although FIGS. 29B and 29C each illustrate an example in which thesecond driver circuit 903 is formed separately and mounted on the firstsubstrate 901, one embodiment of the present invention is not limited tothis structure. The first driver circuit may be separately formed andthen mounted, or only part of the second driver circuit or part of thefirst driver circuit may be separately formed and then mounted.

A connection method of a separately formed driver circuit is notparticularly limited; for example, a chip on glass (COG) method, a wirebonding method, or a method of mounting a tape carrier package (TCP) orthe like can be used. FIG. 29A shows an example in which the seconddriver circuit 903 and the first driver circuit 904 are mounted by a COGmethod; FIG. 29B shows an example in which the second driver circuit 903is mounted by a COG method; and FIG. 29C shows an example in which thesecond driver circuit 903 is mounted as a TCP.

The display device includes, in its category, a panel in which a displayelement is sealed, and a module in which an IC or the like including acontroller is mounted on the panel.

Note that the display device in this specification refers to an imagedisplay device or a display device. The display device may serve as alight source (including a lighting device). Furthermore, the displaydevice also includes the following modules in its category: a module towhich a connector such as an FPC or a TCP is attached; a module having aTCP at the tip of which a printed wiring board is provided; and a modulein which an integrated circuit (IC) is directly mounted on a displayelement by a COG method.

The pixel portion and the first driver circuit provided over the firstsubstrate include a plurality of transistors and any of the transistorswhich are described in the above embodiments can be used.

Examples of the display element provided in the display device include aliquid crystal element and a light-emitting element. An example of theliquid crystal element is an element that controls transmission ornon-transmission of light by optical modulation action of liquidcrystals. The element can be formed by a pair of electrodes and a liquidcrystal layer. The optical modulation action of a liquid crystal iscontrolled by an electric field applied to the liquid crystal (includinga horizontal electric field, a vertical electric field, and an obliqueelectric field). Note that specific examples of the liquid crystalelement include the following: a nematic liquid crystal, a cholestericliquid crystal, a smectic liquid crystal, a discotic liquid crystal, athermotropic liquid crystal, a lyotropic liquid crystal, a low-molecularliquid crystal, a high-molecular liquid crystal, a polymer dispersedliquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, and a banana-shaped liquidcrystal. Examples of the method for driving a liquid crystal include aTN (twisted nematic) mode, an STN (super twisted nematic) mode, an IPS(in-plane-switching) mode, an FFS (fringe field switching) mode, an MVA(multi-domain vertical alignment) mode, a PVA (patterned verticalalignment) mode, an ASV (advanced super view) mode, an ASM (axiallysymmetric aligned micro-cell) mode, an OCB (optically compensatedbirefringence) mode, an ECB (electrically controlled birefringence)mode, an FLC (ferroelectric liquid crystal) mode, an AFLC(anti-ferroelectric liquid crystal) mode, a PDLC (polymer dispersedliquid crystal) mode, a guest-host mode, and a blue-phase mode. Notethat one embodiment of the present invention is not limited thereto, andvarious liquid crystal elements and driving methods can be used. Alight-emitting element includes, in its category, an element whoseluminance is controlled by current or voltage, and specifically includesan inorganic electroluminescent (EL) element and an organic EL element.Further, a display medium whose contrast is changed by an electriceffect, such as electronic ink, can be used. FIG. 30 illustrates anexample of a liquid crystal display device using a liquid crystalelement as a display element.

FIG. 30 shows a cross-sectional view of a vertical electric field modeliquid crystal display device. The liquid crystal display deviceincludes a connection terminal electrode 915 and a terminal electrode916. The connection terminal electrode 915 and the terminal electrode916 are electrically connected to a terminal included in the FPC 918through an anisotropic conductive agent 919.

The connection terminal electrode 915 is formed using the sameconductive film as a first electrode 930. The terminal electrode 916 isformed using the same conductive film as source and drain electrodes oftransistors 910 and 911.

The pixel portion 902 and the first driver circuit 904 which areprovided over the first substrate 901 each include a plurality oftransistors, and the transistor 910 included in the pixel portion 902and the transistor 911 included in the first driver circuit 904 areillustrated as an examples. An insulating film 924 corresponding to theinsulating films 129, 131, and 132 shown in Embodiment 1 is providedover the transistors 910 and 911. Further, an insulating film 934 isprovided over the insulating film 924 in order to improve planarity. Aninsulating film 923 is a nitride insulating film.

In this embodiment, the transistor provided in the pixel 101 shown inEmbodiment 1 can be used as the transistor 910. Further, the transistorprovided in the first driver circuit 104 shown in Embodiment 1 can beused as the transistor 911. Although the transistor 911 includes aconductive film 917, the conductive film 917 is not necessarilyprovided.

A capacitor 936 is formed using an oxide semiconductor film 927, theinsulating films 924 and 934, and the first electrode 930. The oxidesemiconductor film 927 is electrically connected to a capacitor line929. The capacitor line 929 is formed using the same conductive film asgate electrodes of the transistors 910 and 911. Although the capacitordescribed in Embodiment 1 is illustrated as the capacitor 936, any ofthe capacitors in the other embodiments may be used as appropriate.

The transistor 910 provided in the pixel portion 902 is electricallyconnected to the display element to form a display panel. There is noparticular limitation on the kind of the display element as long asdisplay can be performed, and various kinds of display elements can beemployed.

A liquid crystal element 913 is a display element including the firstelectrode 930, a second electrode 931, and a liquid crystal layer 908.An insulating film 932 and an insulating film 933 which serve asalignment films are provided so that the liquid crystal layer 908 isinterposed therebetween. The second electrode 931 is provided on thesecond substrate 906 side. The second electrode 931 overlaps the firstelectrode 930 with the liquid crystal layer 908 provided therebetween.

The first electrode 930 and the second electrode 931 (each of which maybe called a pixel electrode, a common electrode, a counter electrode, orthe like) for applying voltage to the display element may havelight-transmitting properties or light-reflecting properties, whichdepends on the direction in which light is extracted, the position wherethe electrode is provided, and the pattern structure of the electrode.

The first electrode 930 and the second electrode 931 can be formed usingmaterials similar to those of the pixel electrode 121 shown inEmbodiment 1 as appropriate.

A spacer 935 is a columnar spacer obtained by selective etching of aninsulating film and is provided in order to control the distance betweenthe first electrode 930 and the second electrode 931 (a cell gap).Alternatively, a spherical spacer may also be used.

In the case where a liquid crystal element is used as the displayelement, a thermotropic liquid crystal, a low-molecular liquid crystal,a high-molecular liquid crystal, a polymer dispersed liquid crystal, aferroelectric liquid crystal, an anti-ferroelectric liquid crystal, orthe like can be used. Such a liquid crystal material exhibits acholesteric phase, a smectic phase, a cubic phase, a chiral nematicphase, an isotropic phase, or the like depending on a condition.

Alternatively, a liquid crystal exhibiting a blue phase for which analignment film is unnecessary may be used. A blue phase is one of liquidcrystal phases, which is generated just before a cholesteric phasechanges into an isotropic phase while temperature of cholesteric liquidcrystal is increased. Since the blue phase appears only in a narrowtemperature range, a liquid crystal composition in which a chiralmaterial is mixed is used for the liquid crystal layer in order toimprove the temperature range. Note that the alignment film is formedusing an organic resin containing hydrogen, water, or the like, whichmight degrade the electrical characteristics of the transistor in thesemiconductor device of one embodiment of the present invention. Thus,the use of liquid crystal which exhibits a blue phase for the liquidcrystal layer allows the semiconductor device of one embodiment of thepresent invention to be fabricated without an organic resin, so that ahighly reliable semiconductor device can be obtained.

The first substrate 901 and the second substrate 906 are fixed in placeby a sealant 925. As the sealant 925, an organic resin such as athermosetting resin or a photocurable resin can be used. The sealant 925is in contact with the insulating film 924. Note that the sealant 925corresponds to the sealant 905 illustrated in FIGS. 29A to 29C.

The sealant 925 is provided over the insulating film 924. The insulatingfilm 934 is provided on the inner side of the sealant 925. The uppermostlayer of the insulating film 924 is a nitride insulating film, and cansuppress the entry of impurities such as hydrogen and water from theoutside. In contrast, the insulating film 934 has a high moisturepermeability. Thus, when the insulating film 934 is provided on theinner side of the sealant 925 and the sealant 925 is provided over theinsulating film 924, it is possible to suppress the entry of impuritiessuch as hydrogen and water from the outside, and thus to reducevariation in the electrical characteristics of the transistors 910 and911.

In the liquid crystal display device, a black matrix (a light-blockingfilm); an optical member (an optical substrate) such as a polarizingmember, a retardation member, or an anti-reflection member; and the likeare provided as appropriate. For example, circular polarization may beobtained by using a polarizing substrate and a retardation substrate. Inaddition, a backlight, a side light, or the like may be used as a lightsource.

Since the transistor is easily broken owing to static electricity or thelike, a protective circuit for protecting the driver circuit ispreferably provided. The protection circuit is preferably formed using anonlinear element.

FIGS. 31A to 31C illustrate an example of the liquid crystal displaydevice in FIG. 30 in which a common connection portion (pad portion) forbeing electrically connected to the second electrode 931 provided on thesubstrate 906 is formed over the substrate 901.

The common connection portion is provided in a position overlapping thesealant for bonding the substrate 901 and the substrate 906, and iselectrically connected to the second electrode 931 through conductiveparticles contained in the sealant. Alternatively, the common connectionportion is provided in a position not overlapping the sealant (exceptfor the pixel portion) and a paste including conductive particles isprovided separately from the sealant so as to overlap the commonconnection portion, whereby the common connection portion iselectrically connected to the second electrode 931.

The right side of FIG. 31A shows a cross-sectional view of thetransistor 910 in the pixel portion, and the left side of FIG. 31A is across-sectional view of the common connection portion which can beformed through the same steps as the transistor 910. The commonconnection portion in FIG. 31A corresponds to the cross section alongI-J in the top view of the common connection portion illustrated in FIG.31B.

A common potential line 975 is provided over a gate insulating film 922and formed using the same material and through the same steps as asource electrode 971 and a drain electrode 973 of the transistor 910.

Further, the common potential line 975 is covered with the insulatingfilms 924 and 934, and a plurality of openings are formed in theinsulating films 924 and 934 so as to overlap the common potential line975. These openings are formed through the same steps as a contact holewhich connects the first electrode 930 and one of the source electrode971 and the drain electrode 973 of the transistor 910.

Further, the common potential line 975 is connected to a commonelectrode 977 through the openings provided in the insulating films 924and 934. The common electrode 977 is provided over the insulating film934 and formed using the same material and through the same steps as theconnection terminal electrode 915 and the first electrode 930 in thepixel portion.

In this manner, the common connection portion can be manufactured in thesame process as the switching element in the pixel portion 902.

The common electrode 977 is an electrode in contact with the conductiveparticles contained in the sealant, and is electrically connected to thesecond electrode 931 of the second substrate 906.

As illustrated in FIG. 31C, a common potential line 985 may be formedusing the same material and through the same steps as the gate electrodeof the transistor 910.

In the common connection portion in FIG. 31C, the common potential line985 is provided under the gate insulating film 922 and the insulatingfilms 924 and 934, and a plurality of openings are formed in the gateinsulating film 922 and the insulating films 924 and 934 so as tooverlap the common potential line 985. These openings are formed byetching the insulating films 924 and 934 through the same steps as acontact hole which connects the first electrode 930 and one of thesource electrode 971 and the drain electrode 973 of the transistor 910,and further selectively etching the gate insulating film 922.

Further, the common potential line 985 is connected to a commonelectrode 987 through the openings provided in the gate insulating film922 and the insulating films 924 and 934. The common electrode 987 isprovided over the insulating film 934 and formed using the same materialand through the same steps as the connection terminal electrode 915 andthe first electrode 930 in the pixel portion.

As described above, when the transistor and the capacitor shown in theabove embodiment are used, a semiconductor device including thecapacitor with increased charge capacity can be manufactured while theaperture ratio is improved. As a result, the semiconductor device withexcellent display quality can be obtained.

Furthermore, the oxide semiconductor film that is the semiconductor filmin the transistor includes reduced oxygen vacancies and impurities suchas hydrogen; accordingly, the semiconductor device of one embodiment ofthe present invention has good electrical characteristics.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

Embodiment 6

In this embodiment, a structure of an information processor capable ofimage processing and displaying, to which the semiconductor device ofone embodiment of the present invention can be applied, will bedescribed with reference to FIG. 32 and FIGS. 33A1, 33A2, 33B1, and33B2.

Specifically, description is made on an information processor which hasa first mode in which a G signal for selecting a pixel is output at arate of 30 Hz (30 times per second) or more, preferably more than orequal to 60 Hz (60 times per second) and less than 960 Hz (960 times persecond) and a second mode in which the G signal is output at a rate ofmore than or equal to 11.6 μHz (once per day) and less than 0.1 Hz (0.1times per second), preferably more than or equal to 0.28 μHz (once perhour) and less than 1 Hz (once per second).

When a still image is displayed with the information processor of oneembodiment of the present invention, the refresh rate can be set to lessthan 1 Hz, preferably less than or equal to 0.2 Hz. This enableseye-friendly display, i.e., display which causes less eye fatigue of auser or display which does not put strain on the user's eyes. Further, adisplay image can be refreshed at an optimal rate in accordance with thequality of the image displayed on the display portion. Specifically, indisplaying a still image, the refresh rate can be set lower than that indisplaying a smooth moving image; thus, a still image with less flickercan be displayed and power consumption can be reduced.

FIG. 32 shows a block diagram of a structure of an information processorwith a display function of one embodiment of the present invention.

FIGS. 33A1 and 33A2 show block diagrams each showing a structure of adisplay portion of a display device of one embodiment of the presentinvention.

An information processor 600 with a display function, which is describedin this embodiment, includes a display device 640, an arithmetic device620, and an input unit 500 (see FIG. 32).

The display device 640 includes a display portion 630 and a controlportion 610 (see FIG. 32). A primary image signal 625_V and a primarycontrol signal 625_C can be input to the display device 640. In thedisplay device 640, image information can be displayed on the displayportion 630.

The primary image signal 625_V includes grayscale information (also canbe referred to as luminance information) and chromaticity information onan image, for example.

The primary control signal 625_C includes a signal for controlling thetiming of scan operation of the display device 640, for example.

Note that a power supply potential or the like is supplied to thecontrol portion 610 and the display portion 630 in the display device640.

The control portion 610 has a function of controlling the displayportion 630. For example, a secondary image signal 615_V and/or asecondary control signal 615_C are generated in the control portion 610.

The control portion 610 may include a polarity determination circuit,for example. The polarity determination circuit allows the polarity of asignal to be inverted every frame.

The polarity determination circuit may notify the timing at which thepolarity of the secondary image signal 615_V is to be inverted, and thecontrol portion 610 may be configured to invert the polarity of thesecondary image signal 615_V at the notified timing. Note that thepolarity of the secondary image signal 615_V may be inverted in thecontrol portion 610, or may be inverted in the display portion 630 inaccordance with an instruction from the control portion 610.

The polarity determination circuit may include a counter and a signalgeneration circuit, so that the timing of inverting the polarity of thesecondary image signal 615_V is determined by using a synchronizationsignal.

The counter has a function of counting the number of frame periods byusing the pulse of a horizontal synchronizing signal. The signalgeneration circuit has a function of notifying the timing of invertingthe polarity of the secondary image signal 615_V to the control portion610.

This enables the polarity of the secondary image signal 615_V to beinverted every several consecutive frame periods by using information onthe number of frame periods obtained by the counter.

Image information can be included in the secondary image signal 615_V.

For example, the secondary image signal 615_V may be generated from theprimary image signal 625_V in the control portion 610, and then outputfrom the control portion 610.

Alternatively, in the control portion 610, the secondary image signal615_V can be generated so as to have a polarity inverted every frame andan amplitude equal to the difference between the potential of theprimary image signal 625_V and a reference potential Vsc.

The secondary control signal 615_C can include a signal for controllinga first driver circuit (also referred to as a G driver circuit 632) ofthe display portion 630 or a signal for controlling a second drivercircuit (also referred to as an S driver circuit 633) of the displayportion 630.

For example, the control portion 610 may generate the secondary controlsignal 615_C from the primary control signal 625_C including asynchronizing signal such as a vertical synchronizing signal or ahorizontal synchronizing signal.

The secondary control signal 615S includes, for example, a start pulsesignal SP, a latch signal LP, a pulse width control signal PWC, and aclock signal CK.

Specifically, the secondary control signal 615_C includes an S drivercircuit start pulse signal SP, an S driver circuit clock signal CK, alatch signal LP, and the like that control the operation of the S drivercircuit 633. The secondary control signal 615_C can also include a Gdriver circuit start pulse signal SP, a G driver circuit clock signalCK, a pulse width control signal PWC, and the like that control theoperation of the G driver circuit 632.

The display portion 630 includes a pixel portion 631, a first drivercircuit (also referred to as the G driver circuit 632), and a seconddriver circuit (also referred to as the S driver circuit 633).

The pixel portion 631 does not include light with a wavelength shorterthan 420 nm as display light and includes a plurality of pixels 631 parranged at a resolution of 150 ppi or more and wirings that connect theplurality of pixels 631 p. Each of the plurality of pixels 631 p isconnected to at least one of scan lines G and at least one of signallines S. Note that the kinds and number of the wirings depend on thestructure, number, and position of the pixel 631 p.

For example, in the case where the pixels 631 p are arranged in a matrixof x columns and y rows in the pixel portion 631, the signal lines S1 toSx and scan lines G1 to Gy are provided in the pixel portion 631 (seeFIG. 33A1). The plurality of scan lines (G1 to Gy) can supply G signalsto the respective rows. The plurality of signal lines (S1 to Sx) cansupply S signals to the plurality of pixels.

The G driver circuit 632 can control the supply of a G signal 632_G andselect the scan line G (see FIG. 32).

For example, the pixel portion 631 may be divided into a plurality ofregions (specifically, a first region 631 a, a second region 631 b, anda third region 631 c) to be driven (see FIG. 33A2).

Each region can be provided with the plurality of pixels 631 p, theplurality of scan lines G for selecting the pixels 631 p row by row, andthe plurality of signal lines S for supplying S signals 633_S to theselected pixels 631 p.

In addition, a plurality of G driver circuits (specifically, a first Gdriver circuit 632 a, a second G driver circuit 632 b, and a third Gdriver circuit 632 c) may be provided.

The G driver circuit can control the supply of a G signal 632_G andselect the scan lines G in each region (specifically, the scan lines G1to Gj in the first G driver circuit 632 a, the scan lines Gj+1 to G2 jin the second G driver circuit 632 b, and the scan line G2 j+1 to Gy inthe third G driver circuit 632 c).

The G driver circuit outputs the first driving signal (G signal) 632_Gfor selecting the pixel circuit 634 to the pixel circuit 634. The Gdriver circuit 632 has a first mode of outputting the G signal 632_G forselecting a scan line to each scan line at a rate of 30 Hz (30 times persecond) or more, preferably more than or equal to 60 Hz (60 times persecond) and less than 960 Hz (960 times per second) and a second mode ofoutputting the G signal 632_G to each scan line at a rate of more thanor equal to 11.6 μHz (once per day) and less than 0.1 Hz (0.1 times persecond), preferably more than or equal to 0.28 μHz (once per hour) andless than 1 Hz (once per second).

The G driver circuit 632 can be switched between the first mode and thesecond mode to be operated. For example, the G driver circuit 632 can beswitched between the first mode and the second mode with the use of thesecondary control signal 615_C including a mode switching signal or theG driver circuit start pulse signal included in the secondary controlsignal 615_C. Specifically, the frequency of output of the G drivercircuit start pulse signal from the control portion 610 may becontrolled.

The G signal 632_G is generated by the G driver circuit 632. The Gsignals 632_G are output to the pixels 631 p in each row, whereby thepixels 631 p are selected row by row.

The display portion 630 may include the S driver circuit 633. The Sdriver circuit generates a second driver signal (also referred to as anS signal 633_S) from the secondary image signal 615_V and controls thesupply of the S signals 633_S to the signal lines S (specifically S1 toSx).

The S signal 633_S includes grayscale information on an image and thelike. The S signal 633_S is supplied to the pixel 631 p selected by theG signal 632_G.

The pixel portion 631 includes the plurality of pixels 631 p.

The pixel 631 p includes a display element 635 and a pixel circuit 634including the display element 635 (see FIG. 32).

The pixel circuit 634 holds the S signal 633_S supplied and displayssome image data in the display element 635. The structure of the pixelcircuit 634 can be selected in accordance with the kind or the drivingmethod of the display element 635.

FIG. 33B1 illustrates, as an example of the pixel circuit 634, astructure in which a liquid crystal element 635LC is used as the displayelement 635.

The pixel circuit 634 includes a transistor 634 t including a gateelectrode to which the G signal 632_G is input and a first electrode towhich the S signal is input, and the liquid crystal element 635LCincluding a first electrode electrically connected to a second electrodeof the transistor 634 t and a second electrode to which a commonpotential is supplied.

The pixel circuit 634 includes the transistor 634 t for controlling thesupply of the S signal 633_S to the display element 635.

A gate of the transistor 634 t is connected to any one of the scan linesG1 to Gy. One of a source and a drain of the transistor 634 t isconnected to any one of the signal lines S1 to Sx. The other of thesource and the drain of the transistor 634 t is connected to the firstelectrode of the display element 635.

In the pixel 631 p, one transistor 634 t is used as a switching elementfor controlling the input of the S signal 633_S to the pixel 631 p.Alternatively, a plurality of transistors which serve as one switchingelement may be used in the pixel 631 p. In that case, the plurality oftransistors serving as one switching element may be connected to eachother in parallel, in series, or in combination of parallel connectionand series.

Note that the pixel 631 p may further include a capacitor 634 c forholding voltage between the first electrode and a second electrode ofthe liquid crystal element 635LC and another circuit element such as atransistor, a diode, a resistor, a capacitor, or an inductor as needed.A predetermined common potential Vcom is applied to the second electrodeof the display element 635.

The capacitance of the capacitor 634 c may be adjusted as appropriate.For example, in the second mode described later, the capacitor 634 c isprovided in the case where the S signal 633_S is held for a relativelylong period (specifically, 1/60 sec or longer). The capacitance of thepixel circuit 634 can also be adjusted with a structure in which thecapacitor 634 c is not provided. Further, a virtual capacitor may beformed by a structure in which the first electrode and the secondelectrode of the liquid crystal element 635LC are provided to overlapeach other.

FIG. 33B2 illustrates, as another example of the pixel circuit, astructure in which an EL element 635EL is used as the display element635.

A pixel circuit 634EL includes a first transistor 6340 including a gateelectrode to which the G signal 632_G is input, a first electrode towhich the S signal is input, and a second electrode which iselectrically connected to a first electrode of the capacitor 634 c. Thepixel circuit 634EL also includes a second transistor 634 t_2 includinga gate electrode electrically connected to a second electrode of thefirst transistor 6340, a first electrode electrically connected to asecond electrode of the capacitor 634 c, and a second electrodeelectrically connected to a first electrode of the EL element 635EL. Apower supply potential is supplied to the second electrode of thecapacitor 634 c and the first electrode of the second transistor 634t_2, and a common potential is supplied to a second electrode of the ELelement 635EL. Note that the difference between the power supplypotential and the common potential is larger than the voltage at whichthe EL element 635EL starts to emit light.

In the pixel circuit 634, the transistor 634 t controls whether to applythe potential of the signal line S to the first electrode of the displayelement 635.

Note that a transistor including an oxide semiconductor can be suitablyused as the transistor in the display device of one embodiment of thepresent invention. Embodiments 1 and 2 can be referred to for thedetails of the transistor including an oxide semiconductor.

A transistor including an oxide semiconductor film can have leakagecurrent between a source and a drain in an off state (off-state current)much lower than that of a conventional transistor including silicon. Byusing a transistor with extremely low off-state current in a pixelportion of a display portion, frame frequency can be lowered whileflicker is reduced.

Besides the liquid crystal element 635LC, any of a variety of displayelements such as an OLED element generating electroluminescence whenvoltage is applied thereto and electronic ink utilizing electrophoresiscan be used as the display element 635.

For example, the polarized light transmittance of the liquid crystalelement 635LC can be controlled by the potential of the S signal 633_5;thus, gradation can be expressed.

For example, in the case where a transmissive liquid crystal element isused as the display element 635, a light supply portion 650 can beprovided in the display portion 630. The light supply portion 650includes a light source. The control portion 610 controls driving of thelight source in the light supply portion 650. The light supply portion650 supplies light to the pixel portion 631 including the liquid crystalelement, and functions as a backlight.

The light source in the light supply portion 650 can be a cold cathodefluorescent lamp, a light-emitting diode (LED), an OLED element, or thelike.

In particular, the intensity of blue light emitted from the light sourceis preferably weakened compared to that of light of any other color.Since blue light included in light emitted from the light source is notabsorbed by the cornea and lens of the eye and reaches the retina, it ispossible to reduce long-term effects of blue light on the retina (e.g.,age-related macular degeneration), adverse effects of exposure to bluelight until midnight on the circadian rhythm, and the like.Specifically, the light source preferably emits light that does notinclude light with a wavelength of 400 nm or shorter, preferably 420 nmor shorter, and more preferably 440 nm or shorter.

The pixel in the semiconductor device of one embodiment of the presentinvention absorbs and hardly transmits light with the above wavelength.Accordingly, even when a light source emits light with the abovewavelength, the light with the above wavelength can be reduced orblocked by using the semiconductor device of one embodiment of thepresent invention.

The arithmetic device 620 generates the primary image signal 625_V andthe primary control signal 625_C including a mode switching signal.

The mode switching signal may be generated by, the order of a user ofthe information processor 600.

The user of the information processor 600 can give an order to switchdisplay by using the input unit 500. The arithmetic device 620 may beconfigured to be supplied with an image switching signal 500_C and tooutput the primary control signal 625_C including the mode switchingsignal.

The primary control signal 625_C including a mode switching signal issupplied to the control portion 610 in the display device 640, and thesecondary control signal 615_C including the mode switching signal isoutput from the control portion.

For example, when the secondary control signal 615_C including the modeswitching signal for switching the G driver circuit 632 from the secondmode to the first mode is supplied to the G driver circuit 632, the Gdriver circuit 632 is switched from the second mode to the first mode.After that, the G driver circuit 632 outputs the G signal for at leastone frame, and then switched to the second mode.

Specifically, the input unit 500 may be configured to output the imageswitching signal 500_C to the arithmetic device 620 when sensing a pageturning operation.

The arithmetic device 620 generates the primary image signal 625_Vincluding the page turning operation and outputs the primary imagesignal 625_V as well as the primary control signal 625_C including amode switching signal.

The control portion 610 supplied with the primary image signal 625_V andthe primary control signal 625_C supplies the secondary control signal615_C including the mode switching signal and the secondary image signal615_V for executing the page turning operation.

The G driver circuit 632 supplied with the secondary control signal615_C including the mode switching signal is switched from the secondmode to the first mode and outputs the G signal 632_G at a high rate.

The S driver circuit 633 supplied with the secondary image signal 615_Vincluding the page turning operation outputs the S signal 633_Sgenerated from the secondary image signal 615_V to the pixel circuit634.

In this manner, many frame images including the page turning operationcan be rewritten at high rate by the pixels 631 p. As a result, imagesbased on the secondary image signals 615_V for executing the pageturning operation can be displayed smoothly.

The arithmetic device 620 may be configured to determine whether animage based on the primary image signal 625_V output to the displayportion 630 is a moving image or a still image and to output the primarycontrol signal 625_C including the mode switching signal in accordancewith the determination result.

Specifically, the arithmetic device 620 outputs a switching signal forselecting the first mode when the image based on the primary imagesignal 625_V is a moving image, and outputs a switching signal forselecting the second mode when the image based on the primary imagesignal 625_V is a still image.

A method for determining whether the image based on the primary imagesignal is a moving image or a still image is as follows. Signals for oneframe included in the primary image signal 625_V are compared withsignals for the previous frame and the next frame. It is determined thatthe image is a moving image when the difference between the signals isgreater than a predetermined difference, and it is determined that theimage is a still image in other cases.

When the control portion 610 switches the operating mode of the G drivercircuit from one mode to another mode (e.g., from the second mode to thefirst mode), the G driver circuit may be configured to output the Gsignals 632_G one or more predetermined times, and then be switched tothe other mode.

As the input unit 500, a touch panel, a touch pad, a mouse, a fingerjoystick, a trackball, a data glove, or an imaging device can be used,for example In the arithmetic device 620, an electric signal output fromthe input unit 500 can be associated with coordinates of a displayportion. Accordingly, a user can input an instruction for processinginformation displayed on the display portion.

Examples of information input with the input unit 500 by a user areinstructions for dragging an image displayed on the display portion toanother position; for swiping a screen for turning a displayed image anddisplaying the next image; for scrolling a continuous image; forselecting a specific image; for pinching a screen for changing the sizeof a displayed image; and for inputting handwritten characters.

Note that illuminance is the amount of light incident on an irradiationsurface per unit area and unit time, in which the spectral sensitivityof the eye is taken into consideration.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

Embodiment 7

In this embodiment, an information processing method of an informationprocessor using the semiconductor device of one embodiment of thepresent invention will be described with reference to FIGS. 34A and 34B.

Specifically, description is made on a method for generating an imagethat can be displayed on the display portion of the informationprocessor using the semiconductor device of one embodiment of thepresent invention. In particular, description is made on a method ofswitching images in an eye-friendly way when an image displayed on thedisplay portion is changed into another image, that is, a method ofswitching images with less eye fatigue given to a user or a method ofswitching images without strain on the eyes of a user.

FIG. 34A shows a block diagram illustrating a structure of theinformation processor using the semiconductor device of one embodimentof the present invention, and FIG. 34B is a schematic view illustratingimage data.

In one embodiment of the present invention, display images are smoothlyswitched in a display portion of the information processor.

The above structure reduces the strain put on the user's eyes whendisplay images are switched. It is thus possible to provide a novelinformation processing method which enables eye-friendly display of animage including information processed by an arithmetic unit.

A user may feel eye fatigue when images are rapidly switched anddisplayed, for example, when scenes are switched frequently in a movingimage or when a still image is switched to a different still image.

When an image is switched to a different image, it is preferable thatthe images be switched not instantaneously but gradually (smoothly) andnaturally.

For example, when a first still image is switched to a second stillimage, it is preferable to insert a moving image of the first stillimage fading out and/or a moving image of the second still image fadingin between the first still image and the second still image.Alternatively, a moving image obtained by overlapping the first stillimage and the still second image may be inserted so that the secondstill image fades in at the same time when the first still image fadesout (this technique is also referred to as cross-fading). Furtheralternatively, a moving image for displaying the process in which thefirst still image gradually changes into the second still image may beinserted (this technique is also referred to as morphing).

Note that a first still image may be displayed at a low refresh rate,followed by the display of an image for image switching at a highrefresh rate, and then a second still image may be displayed at a lowrefresh rate.

An example of a method for switching between different images A and Bwill be described below.

FIG. 34A shows a block diagram illustrating a structure of a displayportion in which images can be switched. The display portion illustratedin FIG. 34A includes an arithmetic unit 701, a storage unit 702, acontrol unit 703, and a display unit 704.

In a first step, data of the image A and data of the image B input froman external memory device or the like are stored in the storage unit 702by the arithmetic unit 701.

In a second step, the arithmetic unit 701 sequentially generates newimage data based on the data of the image A and the data of the image Bin accordance with a predetermined number by which the image data isdivided.

In a third step, the generated image data is output to the control unit703. The control unit 703 makes the input image data be displayed on thedisplay unit 704.

FIG. 34B shows a schematic view illustrating image data generated whenimages are gradually switched from the image A to the image B.

FIG. 34B shows the case where N pieces of image data (Nis a naturalnumber) are generated to be displayed between the image A and the imageB and each piece of the image data is displayed for f frame periods (fis a natural number). Thus, it takes f×N frames to switch display fromthe image A to the image B.

Here, it is preferable that the above parameters such as N and f can beset freely by a user. The arithmetic unit 701 obtains these parametersin advance and generates image data in accordance with the parameters.

Image data generated for the i-th time (i is an integer of 1 to N) canbe generated by weighting the data of the image A and the data of theimage B and adding the weighted data. For example, when the luminance(gray level) of a pixel displaying the image A is denoted by a and thatof the pixel displaying the image B is denoted by b, the luminance (graylevel) c of the pixel displaying the image data generated for the i-thtime is represented by Formula 1. Note that the gray level means a levelof gradation displayed on the display portion. An image with gradationof only two levels, white and black, can be referred to as a two-levelimage. For example, a display portion of a conventional personalcomputer includes subpixels which display red, green, and blue. Signalsfor expressing gradation of 256 levels are input to the subpixels.

$\begin{matrix}\left\lbrack {{FORMULA}\mspace{14mu} 1} \right\rbrack & \; \\{c = \frac{{\left( {N - i} \right)a} + {ib}}{N}} & (1)\end{matrix}$

When the image data generated by the above method is used for switchingthe display from the image A to the image B, a discontinuous image canbe switched gradually (smoothly) and naturally.

Note that in Formula 1, the case where a=0 in all the pixels correspondsto fade-in by which a black image is gradually switched to the image B.Moreover, the case where b=0 in all the pixels corresponds to fade-outby which the image A is gradually switched to a black image.

Although the method of switching images by temporarily overlapping oftwo images is described above, a method without overlapping operationmay be employed.

In the case where the image A is switched to the image B withoutoverlapping of the two images, a black image may be inserted between theimage A and the image B. At this time, the above method of switchingimages may be employed when the image A changes into a black imageand/or when a black image changes into the image B. Further, an imageinserted between the image A and the image B is not limited to a blackimage, and may be a single-color image such as a white image or amulti-color image different from the image A and the image B.

When an image, particularly a single-color image such as a black imageis inserted between the image A and the image B, the timing of imageswitching can be perceived by a user without causing a feeling ofdiscomfort, so that images can be switched without making the user feelstress.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

Embodiment 8

In this embodiment, a structure of an information processor using thesemiconductor device of one embodiment of the present invention will bedescribed with reference to FIGS. 35A1, 35A2, 35B1, and 35B2, and FIG.36.

FIGS. 35A1, 35A2, 35B1, and 35B2 show an effect of the informationprocessor.

FIG. 36 shows a block diagram illustrating the structure of theinformation processor.

Eye fatigue is classified into two categories: nervous asthenopia andmuscular asthenopia. The eye fatigue is explained using the schematicviews of FIGS. 35A1 and 35A-2.

Nervous asthenopia is caused when a user keeps looking at continuous orblinking light emitted from a display portion for a long time. This isbecause the brightness stimulates and fatigues the retina or nerve ofthe eye or the brain. Frequent blinking of fluorescent light or adisplay portion of a conventional display device, which is calledflicker, causes nervous asthenopia.

Muscular asthenopia is fatigue caused by overuse of the ciliary muscle,which is used to adjust the focus.

FIG. 35A1 shows a schematic view illustrating display on a conventionaldisplay portion. An image is rewritten 60 times per second in display onthe conventional display portion. When a user keeps looking at suchdisplay for a long time, the retina or nerve of the eye or the brain maybe stimulated and eye fatigue might be caused as a result.

In addition, as shown in FIG. 35A2, when the size of each pixel is large(for example, when the resolution is less than 150 ppi), the outline ofa character or the like displayed on the display portion is blurred.When a user keeps looking at the character or the like with a blurredoutline displayed on the display portion for a long time, it remainsdifficult to focus the eye on the character even though the ciliarymuscle constantly moves in order to focus the eye, which might putstrain on the eye.

Methods for quantitatively measuring eye fatigue have been studied. Forexample, critical flicker (fusion) frequency (CFF) is known as anindicator for evaluating nervous asthenopia. Further, accommodationtime, near point distance, and the like are known as indicators forevaluating muscular asthenopia.

Other methods for evaluating eye fatigue include electroencephalography,thermography, counting the number of times of blinking, measuring tearvolume, measuring the speed of contractile response of the pupil, andquestionnaires for surveying subjective symptoms.

In order to solve the above problem, one embodiment of the presentinvention focuses on the illuminance of working environment and the graylevel of a background of image information displayed in a displaydevice. The following embodiment includes one embodiment of the presentinvention which is made by focusing on the illuminance information ofworking environment and the gray level information of a background ofimage information.

A method for processing and displaying image information of oneembodiment of the present invention includes the step of acquiringinformation on the illuminance of environment and information on thegray level of a background of image information displayed on a displayportion; and the step of displaying, with use of the acquiredinformation, the image information on a display portion which includes aplurality of pixels with a resolution of 150 ppi or more and emits lightwith a wavelength of 420 nm or longer. As a result, the imageinformation can be displayed in accordance with the illuminance ofenvironment. It is thus possible to provide a novel method forprocessing and displaying image information, which is capable of imageinformation processing and eye-friendly display.

FIG. 36 shows an example of a block diagram of an information processorcapable of employing the method for processing and displaying imageinformation of one embodiment of the present invention.

An information processor 330 includes an arithmetic unit 311, a storageunit 312, and a transmission path 314. The transmission path 314connects the arithmetic unit 311, the storage unit 312, and aninput/output interface 315 to each other and transmits information. Notethat these units can not be clearly distinguished and one unit alsoserves as another unit or include part of another unit in some cases.For example, a touch panel is an input unit as well as a display unit.

An input/output device 320 is connected to the transmission path 314 viathe input/output interface 315. The input/output device 320 is a devicefor inputting information to an arithmetic device 310 from the outsideor outputting information from the arithmetic device 310.

Examples of the input/output device 320 include a communication device,a network connection device, and a writable external memory device suchas a hard disk or a removable memory.

Examples of an input device 321 include a human interface device such asa keyboard, a pointing device (e.g., a mouse), or a touch panel, acamera such as a digital camera or a digital video camera, a scanner,and a read-only external memory device such as a CD-ROM or a DVD-ROM.For example, a user of the information processor 330 can input a pageturning instruction or the like with use of the input device 321.

A display unit 322, a speaker, a printer, and the like can be connectedas the output device.

The information processor 330 of one embodiment of the present inventionincludes the display unit 322. In particular, display light of thedisplay unit 322 does not include light with a wavelength shorter than420 nm, preferably shorter than 440 nm. A display portion includes aplurality of pixels with a resolution of 150 ppi or more, preferably 200ppi or more. This enables eye-friendly display. Note that in thisspecification, the display light refers to light emitted from orreflected on a display unit of an information processor so that a usercan see displayed images.

The display light of the display unit of one embodiment of the presentinvention is not absorbed by the cornea and lens of the eye and reachesthe retina, and therefore does not include light having long-termeffects on the retina or adverse effects on the circadian rhythm.Specifically, light for displaying images does not include light with awavelength of 400 nm or shorter, preferably 420 nm or shorter, and morepreferably 440 nm or shorter.

The semiconductor device of one embodiment of the present invention canbe used for the information processor 330 of one embodiment of thepresent invention. The pixel in the semiconductor device absorbs andhardly transmits light with the above wavelength. Accordingly, even whena light source emits light with the above wavelength, the light with theabove wavelength can be reduced or blocked by using the semiconductordevice of one embodiment of the present invention.

In addition, the display portion of one embodiment of the presentinvention includes pixels with a resolution of 150 ppi or more,preferably 200 ppi or more, that is, each pixel has a small size. Thisallows muscular asthenopia of the user's eyes to be reduced.

FIGS. 35B1 and 35B2 show schematic views illustrating an effect ofreducing the eye fatigue in the information processor of one embodimentof the present invention.

In the information processor of one embodiment of the present invention,the rate at which a signal for selecting a pixel is output can bechanged. In particular, when a transistor with extremely low off-statecurrent is used in a pixel portion of a display portion, frame frequencycan be lowered while flicker is reduced. For example, an image can berewritten as less frequently as once every five seconds. This enablesthe user to see the same one image as long as possible, so that flickeron the screen recognized by the user is reduced. Thus, stimuli to theretina or nerve of the eye or the brain of the user are reduced, andnervous asthenopia is reduced accordingly (see FIG. 35B1).

Note that a transistor including an oxide semiconductor, particularly atransistor including a CAAC-OS is suitably used as a transistor withextremely low off-state current.

Each pixel in the information processor of one embodiment of the presentinvention has a small size. Specifically, resolution as high as 150 ppior more, preferably 200 ppi or more can be achieved. It is also possibleto display precise and smooth images with a clear outline, which allowsciliary muscles to adjust the focus more easily, and reduces muscularasthenopia of users (see FIG. 35B2). Note that resolution can beexpressed by pixel density (pixel per inch (ppi)). Pixel density is thenumber of pixels per inch. A pixel is a unit composing an image.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

Embodiment 9

The semiconductor device of one embodiment of the present invention canbe applied to a variety of electronic devices (including game machines).Examples of the electronic devices include a television device, amonitor of a computer or the like, a digital camera, a digital videocamera, a digital photo frame, a mobile phone, a game console, aportable game machine, a portable information terminal, an audioreproducing device, and a game machine (e.g., a pachinko machine or aslot machine). Examples of such electronic devices are illustrated inFIGS. 37A to 37C and FIGS. 38A and 38B.

FIG. 37A illustrates a table having a display portion. In the table9000, a display portion 9003 is incorporated in a housing 9001 and animage can be displayed on the display portion 9003. Note that thehousing 9001 is supported by four leg portions 9002. Further, a powercord 9005 for supplying power is provided for the housing 9001.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9003. Thus, the display portion 9003 canhave high display quality.

The display portion 9003 has a touch-input function. When a user touchesbuttons 9004 which are displayed on the display portion 9003 of thetable 9000 with his/her fingers or the like, the user can carry outoperation of the screen and input of information. Further, when thetable may be made to communicate with home appliances or control thehome appliances, the display portion 9003 may function as a controldevice which controls the home appliances by operation on the screen.For example, with the use of a semiconductor device having a touchsensor function or an image sensor function, the display portion 9003can have a touch-input function.

Further, the screen of the display portion 9003 can be placedperpendicular to a floor with a hinge provided for the housing 9001;thus, the table 9000 can also be used as a television device. When atelevision device having a large screen is set in a small room, an openspace is reduced; however, when a display portion is incorporated in atable, a space in the room can be efficiently used.

FIG. 37B illustrates a television device. In a television device 9100, adisplay portion 9103 is incorporated in a housing 9101 and an image canbe displayed on the display portion 9103. Note that the housing 9101 issupported by a stand 9105 here.

The television device 9100 can be operated with an operation switch ofthe housing 9101 or a separate remote controller 9110. Channels andvolume can be controlled with an operation key 9109 of the remotecontroller 9110 so that an image displayed on the display portion 9103can be controlled. Furthermore, the remote controller 9110 may beprovided with a display portion 9107 for displaying data output from theremote controller 9110.

The television device 9100 illustrated in FIG. 37B is provided with areceiver, a modem, and the like. With the receiver, general televisionbroadcasts can be received in the television device 9100. Further, whenthe television device 9100 is connected to a communication network bywired or wireless connection via the modem, one-way (from a transmitterto a receiver) or two-way (between a transmitter and a receiver orbetween receivers) data communication can be performed.

The semiconductor device described in any of the above embodiments canbe used for the display portions 9103 and 9107. Thus, the television setcan have high display quality.

FIG. 37C illustrates a computer 9200, which includes a main body 9201, ahousing 9202, a display portion 9203, a keyboard 9204, an externalconnection port 9205, a pointing device 9206, and the like.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9203. Thus, the computer 9200 can havehigh display quality.

The display portion 9203 has a touch-input function. When a user touchesbuttons and the like which are displayed on the display portion 9203with his/her fingers or the like, the user can carry out operation onthe screen and input of information. Furthermore, information can beinput using a keyboard or voice input.

FIGS. 38A and 38B illustrate a foldable tablet terminal. In FIG. 38A,the tablet terminal is opened and includes a housing 9630, a displayportion 9631 a, a display portion 9631 b, a display-mode switchingbutton 9034, a power button 9035, a power-saving-mode switching button9036, a clip 9033, and an operation button 9038.

The semiconductor device described in any of the above embodiments canbe used for the display portion 9631 a and the display portion 9631 b.Thus, the display quality of the tablet terminal can be improved.

Part of the display portion 9631 a can be a touch panel region 9632 aand data can be input when a displayed operation key 9638 is touched.Although a structure in which a half region in the display portion 9631a has only a display function and the other half region also has a touchpanel function is illustrated as an example, the structure of thedisplay portion 9631 a is not limited thereto. The whole area of thedisplay portion 9631 a may have a touch panel function. For example, thewhole area of the display portion 9631 a can display keyboard buttonsand serve as a touch screen while the display portion 9631 b can be usedas a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touch panel region 9632 b. When a keyboard display switching button9639 displayed on the touch panel is touched with a finger, a stylus, orthe like, a keyboard can be displayed on the display portion 9631 b.

Touch input can be performed concurrently on the touch panel regions9632 a and 9632 b.

The display-mode switching button 9034 can switch display orientation(e.g., between landscape mode and portrait mode) and select a displaymode (switch between monochrome display and color display), for example.The power-saving-mode switching button 9036 can control displayluminance in accordance with the amount of external light in use of thetablet terminal detected by an optical sensor incorporated in thetablet. The tablet terminal may include another detection device such asa sensor for detecting orientation (e.g., a gyroscope or an accelerationsensor) in addition to the optical sensor.

Although the display portion 9631 a and the display portion 9631 b havethe same display area in FIG. 38A, one embodiment of the presentinvention is not limited to this example. The display portion 9631 a andthe display portion 9631 b may have different areas or different displayquality. For example, one of them may be a display panel that candisplay higher-definition images than the other.

In FIG. 38B, the tablet terminal is folded and includes the housing9630, a solar cell 9633, and a charge and discharge control circuit9634. Note that FIG. 38B illustrates an example in which the charge anddischarge control circuit 9634 includes the battery 9635 and the DCDCconverter 9636.

Since the tablet terminal can be folded in two, the housing 9630 can beclosed when not in use. Thus, the display portions 9631 a and 9631 b canbe protected, thereby providing a tablet with high endurance and highreliability for long-term use.

In addition, the tablet terminal illustrated in FIGS. 38A and 38B canhave a function of displaying various kinds of data (e.g., a stillimage, a moving image, and a text image), a function of displaying acalendar, a date, the time, or the like on the display portion, atouch-input function of operating or editing the data displayed on thedisplay portion by touch input, a function of controlling processing bya variety of kinds of software (programs), and the like.

The solar cell 9633, which is attached on the surface of the tabletterminal, supplies electric power to a touch screen, a display portion,an image signal processor, and the like. Note that the solar cell 9633can be provided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently. The use of a lithium ionbattery as the battery 9635 is advantageous in downsizing or the like.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 38B are described with reference to blockdiagrams of FIGS. 39A to 39C. FIG. 39A illustrates the solar cell 9633,the battery 9635, the DCDC converter 9636, a DCDC converter 9637,switches SW1 to SW3, and a load (e.g., the display portion 9631), andthe battery 9635, the DCDC converter 9636, the DCDC converter 9637, andthe switches SW1 to SW3 correspond to the charge/discharge controlcircuit 9634 in FIG. 38B.

First, an example of the operation in which power is generated by thesolar cell 9633 will be described. The voltage of power generated by thesolar cell 9633 is raised or lowered by the DCDC converter 9636 so thata voltage for charging the battery 9635 is obtained. When the load(e.g., the display portion 9631) is operated with the power from thesolar cell 9633, the switch SW1 is turned on and the voltage of thepower is raised or lowered by the DCDC converter 9637 to a voltageneeded for operating the load (e.g., the display portion 9631). Inaddition, when no power is supplied to the load (e.g., the displayportion 9631), the switch SW1 is turned off and the switch SW2 is turnedon so that the battery 9635 is charged.

In the case where power is always supplied to the load (e.g., thedisplay portion 9631) through the battery 9635, the switch SW1 may beomitted as illustrated in FIG. 39B.

Further, in the case where the voltage supplied to the load is in therange equivalent to the voltage of the battery 9635, the DCDC converter9637 may also be omitted as illustrated in FIG. 39C.

Note that the solar cell 9633 is described as an example of a powergeneration means; however, without limitation thereon, the battery 9635may be charged using another power generation means such as apiezoelectric element or a thermoelectric conversion element (Peltierelement). For example, the battery 9635 may be charged with anon-contact power transmission module that transmits and receives powerwirelessly (without contact) to charge the battery or with a combinationof other charging means.

Note that this embodiment can be combined with any of the otherembodiments shown in this specification.

This application is based on Japanese Patent Application serial No.2012-288973 filed with Japan Patent Office on Dec. 28, 2012, andJapanese Patent Application serial No. 2013-049261 filed with JapanPatent Office on Mar. 12, 2013, the entire contents of which are herebyincorporated by reference.

REFERENCE NUMERALS

100: pixel portion 101: pixel 102: substrate 103: transistor 104: drivercircuit 105: capacitor 106: driver circuit 107: scan line 108: liquidcrystal element 109: signal line 110: base insulating film 111:semiconductor film 113: conductive film 115: capacitor line 117: opening118: nitride insulating film 119: semiconductor film 121: pixelelectrode 127: gate insulating film 129: insulating film 130: insulatingfilm 131: insulating film 132: insulating film 135: boundary 154:counter electrode 188 a: oxide semiconductor film 188 b: oxidesemiconductor film 199 a: oxide semiconductor film 199 b: oxidesemiconductor film 199 c: oxide semiconductor film 223: transistor 227:gate electrode 229: wiring 231: semiconductor film 233: wiring 241:conductive film 310: arithmetic device 311: arithmetic unit 312: storageunit 314: transmission path 315: input/output interface 320:input/output device 321: input device 322: display unit 330: informationprocessor 500: input unit 500_C: signal 600: information processor 610:control portion 615_C: secondary control signal 615_V: secondary imagesignal 620: arithmetic device 625_C: primary control signal 625_V:primary image signal 630: display portion 631: pixel portion 631 a:region 631 b: region 631 c: region 631 p: pixel 632: G driver circuit632_G: G signal 632 a: G driver circuit 632 b: G driver circuit 632 c: Gdriver circuit 633: S driver circuit 633_5: S signal 634: pixel circuit634 c: capacitor 634EL: pixel circuit 634 t: transistor 6340: transistor634 t_2: transistor 635: display element 635EL: EL element 635LC: liquidcrystal element 640: display device 650: light supply portion 701:arithmetic unit 702: storage unit 703: control unit 704: display unit901: substrate 902: pixel portion 903: driver circuit 904: drivercircuit 905: sealant 906: substrate 908: liquid crystal layer 910:transistor 911: transistor 913: liquid crystal element 915: connectionterminal electrode 916: terminal electrode 917: conductive film 918: FPC918 b: FPC 919: anisotropic conductive agent 922: gate insulating film923: insulating film 924: insulating film 925: sealant 926: capacitor927: oxide semiconductor film 929: capacitor line 930: electrode 931:electrode 932: insulating film 933: insulating film 934: insulating film935: spacer 936: capacitor 971: source electrode 973: drain electrode975: common potential line 977: common electrode 985: common potentialline 987: common electrode 9000: table 9001: housing 9002: leg portion9003: display portion 9004: button 9005: power cord 9033: clip 9034:switching button 9035: power button 9036: switching button 9038:operation button 9100: television device 9101: housing 9103: displayportion 9105: stand 9107: display portion 9109: operation key 9110:remote controller 9200: computer 9201: main body 9202: housing 9203:display portion 9204: keyboard 9205: external connection port 9206:pointing device 9630: housing 9631: display portion 9631 a: displayportion 9631 b: display portion 9632 a: region 9632 b: region 9633:solar cell 9634: charge/discharge control circuit 9635: battery 9636:DCDC converter 9637: DCDC converter 9638: operation key 9639: switchingbutton

1. A semiconductor device comprising: a transistor comprising: a firstoxide semiconductor layer; a source electrode layer and a drainelectrode layer; a first insulating film over the first oxidesemiconductor layer; and a gate electrode layer over the firstinsulating film, a capacitor comprising; a second oxide semiconductorlayer; a dielectric over the second oxide semiconductor layer; and atransparent conductive film over the dielectric; a display elementcomprising; a first pixel electrode; and a second pixel electrode; and awiring being in contact with the second oxide semiconductor layer,wherein each of the transparent conductive film and the first pixelelectrode comprises a first material, wherein each of the sourceelectrode layer, the drain electrode layer and the wiring comprises asecond material, and wherein each material of the first oxidesemiconductor layer and the second oxide semiconductor layer comprisesIn, Zn and O.
 2. The semiconductor device according to claim 1, whereinthe wiring overlaps and the second oxide semiconductor layer overlapeach other.
 3. The semiconductor device according to claim 2, whereinthe wiring and the transparent conductive film overlap each other, andwherein the dielectric is interposed between the wiring and thetransparent conductive film.
 4. The semiconductor device according toclaim 1, further comprising: a second insulating film over the gateelectrode layer; and a third insulating film over the second insulatingfilm, wherein the second oxide semiconductor layer and each of thesecond insulating film and the third insulating film overlap each other,and wherein the dielectric comprises the second insulating film and thethird insulating film.
 5. The semiconductor device according to claim 4,wherein the second insulating film has a single-layer structure or alayered structure including an oxide insulating material selected fromsilicon oxide, silicon oxynitride, aluminum oxide, hafnium oxide,gallium oxide, and a Ga—Zn-based metal oxide.
 6. The semiconductordevice according to claim 4, wherein the third insulating film has asingle-layer structure or a layered structure including a nitrideinsulating material selected from silicon nitride oxide, siliconnitride, aluminum nitride, and aluminum nitride oxide.
 7. Thesemiconductor device according to claim 1, further comprising: a secondinsulating film over the gate electrode layer, the second insulatingfilm being in contact with the second oxide semiconductor layer; and athird insulating film over the second insulating film, wherein thesecond insulating film and the third insulating film overlap the secondoxide semiconductor layer, and wherein the dielectric includes thesecond insulating film and the third insulating film.
 8. Thesemiconductor device according to claim 1, further comprising: a secondinsulating film over the gate electrode layer; and a third insulatingfilm over the second insulating film, the third insulating film being incontact with the second oxide semiconductor layer, wherein the thirdinsulating film overlaps the second oxide semiconductor layer, andwherein the dielectric includes the third insulating film.
 9. Thesemiconductor device according to claim 1, wherein said each material ofthe first oxide semiconductor layer and the second oxide semiconductorlayer further comprises gallium.
 10. The semiconductor device accordingto claim 1, wherein the material of the second oxide semiconductor layercomprises one or more additive elements selected from hydrogen, boron,nitrogen, fluorine, aluminum, phosphorus, arsenic, indium, tin,antimony, and a rare gas element.
 11. The semiconductor device accordingto claim 1, wherein the first oxide semiconductor layer and the secondoxide semiconductor layer each has an energy gap of 2.0 eV or more. 12.The semiconductor device according to claim 1, wherein the transparentconductive film is electrically connected to one of the source electrodelayer and the drain electrode layer.
 13. The semiconductor deviceaccording to claim 1, wherein the display element is an organicelectroluminescent element.
 14. The semiconductor device according toclaim 1, wherein the display element is a liquid crystal element.